fix(ui): decode the font atlas's SDF in atlas pixels, so text edges ramp over one screen pixel

Shaders
- ui-text and ui-fused turned a sample of the R8_UNORM atlas into a
  distance with (128/255 - sdf) * 32. stb_truetype writes kOnEdgeValue +
  kPixelDistScale * distance into each byte, so an atlas pixel is
  kPixelDistScale / 255 of a sample; 32 made every distance 1024/255
  (about 4) times too large, and the coverage ramp, meant to span one
  screen pixel, spanned a quarter of one: nearly every edge pixel came out
  fully on or fully off, and text looked pixelated at every size. On an
  RTX 4090, UIFallbackParity's 22 px label had 98 partly covered pixels
  where stb_truetype's own rasterizer has 415. Wrong since the v3 UI
  (1f56973), and copied into the fused kernel (574242d).
- The decode now lives once, in ui-shared.glsl: UI_SDF_ON_EDGE and
  UI_SDF_PIXEL_DIST_SCALE are FontAtlas's two constants in the encoder's
  own units, uiSdfDistance decodes a sample, uiGlyphRamp works out a
  glyph's ramp when it is loaded and uiGlyphCoverage the coverage per
  pixel. Both kernels call them, and a copy of either should; the UI_
  prefix keeps them clear of a copied kernel's own names.
- stb_truetype truncates, so a byte b stands for [b, b + 1) of the encoded
  value and the decode takes its middle: decoding b pulled every outline
  1/64 atlas pixel inwards.
- The ramp is capped at what a byte holds, about 4 atlas pixels each way
  of the outline. With the corrected decode a wider one left every glyph
  quad's padding translucent below 4 px.
- The UV span is taken with abs(): a glyph turned by swapping its UVs got
  a negative scale, a ramp of 0.0001 and hard edges.
- A glyph scaled differently along x and y ramps along the distance's
  gradient, from two one-texel differences. With the corrected decode, one
  width for both axes, the more minified one's, would leave a row stretched
  three times with stems three pixels soft. Glyphs scaled alike, as
  ShapeText's are, keep one multiply-add per pixel, the ramp's reciprocal
  worked out with the glyph.
- The atlas size is queried rather than FontAtlas::kAtlasSize hard-coded
  as 1024.

WebGPU
- The WGSL text kernel decoded the distance right but ramped over one
  atlas pixel whatever the size: half a screen pixel at 16 px, two at
  64 px. It now repeats ui-shared.glsl's decode, cap, abs and gradient
  path, and renders TextAntialiasing's scene as Vulkan does (run through
  Deno's WebGPU: 1.02, 1.07 and 1.09 times the reference's edge pixels,
  where the old kernel gave 0.55, 2.19 and 0.89).

FontAtlas
- kOnEdgeValue and kPixelDistScale say the kernels carry copies of them.

Tests
- TextAntialiasing draws one string at 14 px, 64 px and 14 px stretched
  three times, a 'g' turned by its UVs and empty atlas texels shrunk to
  12.8 atlas pixels per screen pixel, through DispatchText and again
  through DispatchFused, and holds them to stb_truetype's antialiased
  rasterizer, to the mirror image and to nothing. On master the two plain
  rows have 0.31 and 0.21 times the reference's edge pixels; with the
  decode fixed alone the stretched row had 1.62 times, the turned 'g' was
  127 codes off its mirror and the empty texels drew 48. Now: 1.02, 1.07
  and 1.09 times, 0 and 0, on the 4090, on llvmpipe and on descriptor
  sets. It skips a swapchain that isn't 8-bit, which the capture can't
  read, and captures the first frame.
- FontAtlasDecode reads ui-shared.glsl and dom-webgpu.js and holds their
  copies of the two constants to FontAtlas's. No GPU.

Docs
- AGENTS.consumer.md has a custom text shader decode through the helpers
  and gives the encoding; README says so in a line; AGENTS.md lists the
  tests and where the decode lives.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -70,9 +70,9 @@ Tests are declared in `project.cpp`, not discovered, through the `LibraryTest(na
 
 A test is an `int main()`: exit 0 passes, nonzero fails, 77 skips (for a missing capability, never for a failed check). Each runs in its own process, in parallel, with a 60 s default timeout, and the output of a failed, crashed or timed-out test is in `build/test-logs/<name>.log`; a later pass does not delete the log, so check its timestamp. What the tests need, in three groups:
 
-- Pure logic, no GPU: they stage `Device`'s statics or feed synthetic data to the decision functions that were factored out for them (`Device::ResolveFeatures`, `ChooseDevice`, `CheckShaderCapabilities`, `GetMemoryType`, `PreferDirectDeviceWrite`, `PipelineCacheDataCompatible`, `AlignMappedFlushRange`, `BuildMipChainFinalBarriers`, `SwapchainStageUnion`, `BuildTlasWriteBarrier`, `UIRenderer::ClipFlags`, `Window::SwapchainMinImageCount`, `WorkaroundNvidiaAS::Patch`, `DescriptorHeapVulkan::ComputeLayout`, `FontAtlas::Packer`, the font and input-field helpers). `PushConstantRewrite` and `UIFusedShader` compile GLSL with glslang and check the SPIR-V with spirv-val at runtime, and `ExampleShaders` compiles every GLSL shader under `examples/` the way crafter-build would, the descriptor-set build included, so a shader contract change that breaks an example fails here without building the example; `MouseScroll` and `PointerEnterLeave` drive the Wayland listener callbacks directly, and `TitleWayland` reads the requests a window sends off a `wl_display` connected to a socketpair.
+- Pure logic, no GPU: they stage `Device`'s statics or feed synthetic data to the decision functions that were factored out for them (`Device::ResolveFeatures`, `ChooseDevice`, `CheckShaderCapabilities`, `GetMemoryType`, `PreferDirectDeviceWrite`, `PipelineCacheDataCompatible`, `AlignMappedFlushRange`, `BuildMipChainFinalBarriers`, `SwapchainStageUnion`, `BuildTlasWriteBarrier`, `UIRenderer::ClipFlags`, `Window::SwapchainMinImageCount`, `WorkaroundNvidiaAS::Patch`, `DescriptorHeapVulkan::ComputeLayout`, `FontAtlas::Packer`, the font and input-field helpers). `PushConstantRewrite` and `UIFusedShader` compile GLSL with glslang and check the SPIR-V with spirv-val at runtime, and `ExampleShaders` compiles every GLSL shader under `examples/` the way crafter-build would, the descriptor-set build included, so a shader contract change that breaks an example fails here without building the example; `FontAtlasDecode` reads `ui-shared.glsl` and `dom-webgpu.js` and holds their copies of the SDF encoding constants to `FontAtlas`'s; `MouseScroll` and `PointerEnterLeave` drive the Wayland listener callbacks directly, and `TitleWayland` reads the requests a window sends off a `wl_display` connected to a socketpair.
 - A headless device: `Device::Initialize()` and no `Window` (`HeapSlotReuse`, `DeferredDeletion`, `MeshLifetime`, `ImageStagingRelease`, `BLASBuildOptions`, the `TLAS*` tests, ...). Headless still means a Wayland display on Linux. Most of the RT ones check `Device::rayTracingSupported` and return 77 without it; `BLASBuildOptions`, `TLASInstanceDirtyTracking` and `MeshDecompressStagingRelease` do not, and crash on a GPU without ray tracing (`Device::RequireRayTracing` throws out of `main`). `DeviceSelection` skips with one GPU.
-- A window and a live compositor: `FrameLoopSync`, `SetupCmdBufferReuse`, `UIFallbackParity`, `SwapchainTransferDst`, `WindowUpdateClock`, `HeapASRead`, `HeapDynamicIndexHit`, `RTPassPushData`, `WindowOptionalGlobals` (hides xdg-decoration, viewporter and fractional scale from the window in turn), `SwapchainImageCount` (every present mode, and swapchains with more images than frames in flight, in child processes on llvmpipe where the host's GPU never makes one), and `FeatureRequests` for its retry case. They render, read the frame back with `Window::CaptureNextFrame` (a PNG, decoded with a private `STB_IMAGE_STATIC` copy of stb_image) or a transfer pass of their own, and check pixels.
+- A window and a live compositor: `FrameLoopSync`, `SetupCmdBufferReuse`, `UIFallbackParity`, `SwapchainTransferDst`, `WindowUpdateClock`, `HeapASRead`, `HeapDynamicIndexHit`, `RTPassPushData`, `WindowOptionalGlobals` (hides xdg-decoration, viewporter and fractional scale from the window in turn), `SwapchainImageCount` (every present mode, and swapchains with more images than frames in flight, in child processes on llvmpipe where the host's GPU never makes one), `TextAntialiasing` (text through `DispatchText` and `DispatchFused` against stb_truetype's own antialiased rasterizer, at 14 and 64 px and stretched, plus a glyph mirrored by its UVs and empty texels shrunk below 4 px), and `FeatureRequests` for its retry case. They render, read the frame back with `Window::CaptureNextFrame` (a PNG, decoded with a private `STB_IMAGE_STATIC` copy of stb_image) or a transfer pass of their own, and check pixels.
 
 Conventions the existing tests follow:
 
@@ -83,7 +83,7 @@ Conventions the existing tests follow:
 - Files go under `fs::temp_directory_path()`, named per test.
 - `HeapASRead --no-workaround` is the manual probe for whether a new NVIDIA driver still has the descriptor-heap AS fault; run it after a driver update and update the README's status note.
 
-A bug fix ships with a test that fails before the fix and passes after it. A change to a UI kernel keeps `UIFallbackParity` green: the heap and descriptor-set builds of one source must stay pixel-identical. A category drawn fused must also match the same category drawn standalone, but no test compares those two (`UIFusedShader` only pins the push-constant layout): check it by eye, or add the comparison.
+A bug fix ships with a test that fails before the fix and passes after it. A change to a UI kernel keeps `UIFallbackParity` green: the heap and descriptor-set builds of one source must stay pixel-identical. A category drawn fused must also match the same category drawn standalone. `TextAntialiasing` compares the two for text, and keeps the text's edge ramp at one screen pixel; for quads, circles and images no test does (`UIFusedShader` only pins the push-constant layout): check those by eye, or add the comparison.
 
 Cross targets: `crafter-build test --target=x86_64-w64-mingw32` needs `wine` on the sandbox's `PATH`, which the Flatpak does not have; a MinGW import library for `vulkan-1`, which the toolchain does not ship; `-luuid`, which the Win32 link flags lack (#184); and a definition of `__libc_single_threaded`, which the MinGW build of Crafter.Asset references. The Win32 tests have been run with an import library made by `llvm-dlltool` from the `vk*` symbols `llvm-nm -u` lists for the MinGW `libCrafter.Graphics.a`, and an archive defining `char __libc_single_threaded = 0;`, both under the gitignored `build/`. A temporary edit of `project.cpp` added `-L` to that directory, both libraries and `-luuid` to the Win32 tests' link flags and cleared their `requires_`; the tests were built with `--runner=cmd:true`, and each exe ran from its bin directory under the host's wine in a scratch `WINEPREFIX`. For an aarch64 device, `crafter-build test --target=...` tries qemu, which has no Vulkan: build with the sysroot, copy the test's whole bin directory (binary, `*.spv`, `*.sets.spv`, fonts) to the device and run it there, unlocked and with the window visible, since a locked phone's compositor sends no frame callbacks and the FIFO present then blocks forever.
 
@@ -150,6 +150,7 @@ These were decided deliberately. Do not undo them without the maintainer's say-s
 - `BuildTlasWriteBarrier`'s source access mask carries no read bit on purpose: synchronization validation 1.4.363 otherwise reports two builds in one command buffer as a write-after-write hazard.
 - NVIDIA, as of driver `615.78.08`: reading a TLAS through the heap still faults (worked around); dynamic descriptor indexing in hit shaders works there and faulted on `610.43.02`, so the Sponza pattern (a spec-constant slot, a dynamic index within the resource) stays the portable one. `HeapDynamicIndexHit` skips on the older drivers.
 - The README links `WAVEFRONT-DESIGN.md`, which was removed with the stale docs (`15ce47f`); the wavefront tracer is described in the README's WebGPU section and in `additional/dom-webgpu.js`.
+- The text kernels decode `FontAtlas`'s SDF in one place, `ui-shared.glsl` (`UI_SDF_ON_EDGE`, `UI_SDF_PIXEL_DIST_SCALE`, `uiGlyphRamp`, `uiGlyphCoverage`), and the WGSL text kernel in `dom-webgpu.js` repeats it; `FontAtlasDecode` holds both copies of the constants to `FontAtlas`'s, and nothing renders the WGSL, so check a change to it by hand (`examples/HelloUI` built with `--target=wasm32-wasip1`, in a browser, or the kernel run through Deno's WebGPU). The WGSL lives in `String.raw` literals: a backtick in a WGSL comment ends the literal and breaks the whole backend, which only `node --check additional/dom-webgpu.js` notices.
 - Crafter.Math's misspelled identifiers (`Quanternion*`, `MulitplyAdd`, `CollumSize`) are its API; do not correct them here.
 - Dependency notes: crafter-build 3.0.0 or newer keeps a section at the end of this file holding the `AGENTS.consumer.md` of every dependency that ships one (2.2.0 leaves the file alone), rewritten on every `crafter-build` and `crafter-build test`. As of that date Crafter.Math's notes are on its `master`, Crafter.Asset's only on its `claude/glb-loader` branch, and Crafter.Event has none, so a plain build adds Math's and a `--local` build whatever the sibling checkouts hold. Edits inside the section's markers are lost on the next build; a change to those notes belongs in their repositories. Commit the section when it changes.