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>

shaders/ui-shared.glsl

@@ -157,6 +157,93 @@ float uiSdRoundRect(vec2 p, vec2 halfSize, vec4 r) {
     return min(max(q.x, q.y), 0.0) + length(max(q, 0.0)) - r.x;
 }
 
+// ─── SDF text (FontAtlas) ───────────────────────────────────────────────
+// FontAtlas keeps each glyph as a signed distance field: stb_truetype writes
+// kOnEdgeValue + kPixelDistScale * d into each texel, d being the distance
+// from the outline in atlas pixels, positive inside, truncated to a byte and
+// clamped to 0..255. These are FontAtlas's two constants in the encoder's own
+// units; the FontAtlasDecode test holds them, and the WGSL copy in
+// dom-webgpu.js, to FontAtlas's. ui-text and the fused kernel's text phase
+// decode through the helpers below, and so should a copy of either.
+const float UI_SDF_ON_EDGE          = 128.0;  // FontAtlas::kOnEdgeValue
+const float UI_SDF_PIXEL_DIST_SCALE = 32.0;   // FontAtlas::kPixelDistScale
+
+// The widest edge ramp, in atlas pixels, the stored distances can carry. A
+// byte holds from (UI_SDF_ON_EDGE - 0.5) / UI_SDF_PIXEL_DIST_SCALE outside
+// the outline to (255.5 - UI_SDF_ON_EDGE) / UI_SDF_PIXEL_DIST_SCALE inside,
+// about 4 atlas pixels each way, and a ramp wider than twice the nearer end
+// never reaches 0 there: below about 4 px every glyph quad's padding, all 0,
+// would show as a translucent box. For text that small a ramp narrower than
+// one screen pixel is the lesser evil.
+const float UI_SDF_MAX_BAND = 2.0 * min(UI_SDF_ON_EDGE - 0.5, 255.5 - UI_SDF_ON_EDGE)
+                            / UI_SDF_PIXEL_DIST_SCALE;
+
+// Distance from the outline in atlas pixels, negative inside, for an atlas
+// sample (0..1). stb_truetype truncates, so byte b stands for [b, b + 1) of
+// the encoded value and b + 0.5 is its unbiased middle: decoding b itself
+// pulls every outline 1/64 atlas pixel inwards.
+float uiSdfDistance(float sdf) {
+    return (UI_SDF_ON_EDGE - (sdf * 255.0 + 0.5)) / UI_SDF_PIXEL_DIST_SCALE;
+}
+
+// What a glyph's per-pixel coverage needs, worked out once per glyph:
+//   atlasPerScreen  the atlas pixels one screen pixel spans along x and y;
+//   coverage        (k, b) of coverage = clamp(k * sdf + b, 0, 1) for a glyph
+//                   scaled alike along x and y (within 1%), as ShapeText's
+//                   always are: a ramp one screen pixel wide folded into one
+//                   multiply-add. (0, 0) for one that isn't, whose ramp is
+//                   then worked out per pixel.
+struct UIGlyphRamp {
+    vec2 atlasPerScreen;
+    vec2 coverage;
+};
+
+// The atlas's size in texels: queried rather than taken from
+// FontAtlas::kAtlasSize, so the two can't drift apart.
+vec2 uiGlyphAtlasSize(uint texSlot, uint samplerSlot) {
+    return vec2(textureSize(sampler2D(uiTextures[texSlot], uiSamplers[samplerSlot]), 0));
+}
+
+// `uv` is the GlyphItem's (u0, v0, u1, v1), `invRectSize` 1 / its (w, h).
+UIGlyphRamp uiGlyphRamp(vec4 uv, vec2 invRectSize, vec2 atlasSize) {
+    UIGlyphRamp ramp;
+    // abs: a glyph turned by swapping its UVs keeps its scale.
+    ramp.atlasPerScreen = abs(uv.zw - uv.xy) * atlasSize * invRectSize;
+    ramp.coverage = vec2(0.0);
+    float lo = min(ramp.atlasPerScreen.x, ramp.atlasPerScreen.y);
+    float hi = max(ramp.atlasPerScreen.x, ramp.atlasPerScreen.y);
+    if (hi <= lo * 1.01) {
+        // 0.5 - uiSdfDistance(sdf) / band, the band one screen pixel.
+        float inv = 1.0 / (UI_SDF_PIXEL_DIST_SCALE * clamp(hi, 0.0001, UI_SDF_MAX_BAND));
+        ramp.coverage = vec2(255.0 * inv, 0.5 - (UI_SDF_ON_EDGE - 0.5) * inv);
+    }
+    return ramp;
+}
+
+// The glyph's coverage at `uv` in the atlas. The slots must be dynamically
+// uniform, as push constants are: there is no nonuniformEXT here, which lets
+// the compiler hoist the descriptor load out of the per-pixel glyph loop.
+float uiGlyphCoverage(uint texSlot, uint samplerSlot, vec2 uv, UIGlyphRamp ramp, vec2 atlasSize) {
+    float sdf = texture(sampler2D(uiTextures[texSlot], uiSamplers[samplerSlot]), uv).r;
+    if (ramp.coverage.x != 0.0) return clamp(ramp.coverage.x * sdf + ramp.coverage.y, 0.0, 1.0);
+
+    // Scaled differently along x and y: one screen pixel spans
+    // atlasPerScreen.x atlas pixels across a vertical edge and
+    // atlasPerScreen.y across a horizontal one, so the ramp's width follows
+    // the direction the distance changes in, from one-texel differences. One
+    // width for both would be several pixels wide across one of them.
+    vec2 texel = 1.0 / atlasSize;
+    vec2 gradient = vec2(
+        texture(sampler2D(uiTextures[texSlot], uiSamplers[samplerSlot]), uv + vec2(texel.x, 0.0)).r,
+        texture(sampler2D(uiTextures[texSlot], uiSamplers[samplerSlot]), uv + vec2(0.0, texel.y)).r) - sdf;
+    float len = length(gradient);
+    // No gradient where the field is clamped, 4 atlas pixels from any
+    // outline, where the coverage is 0 or 1 at any width up to the cap.
+    float band = len > 0.0 ? length(gradient / len * ramp.atlasPerScreen)
+                           : max(ramp.atlasPerScreen.x, ramp.atlasPerScreen.y);
+    return clamp(0.5 - uiSdfDistance(sdf) / clamp(band, 0.0001, UI_SDF_MAX_BAND), 0.0, 1.0);
+}
+
 // ─── shared-memory cooperative tile culling ─────────────────────────────
 // Each shader still dispatches one 8×8 workgroup per screen tile, but rather
 // than have all 64 threads walk the FULL item list — re-reading every item