perf(ui): gate per-pixel clip compares behind a flags bit (#50)

uiResolveScreenPixel ran four float compares against hdr.clipRectPx for
every pixel of every UI dispatch, even though the clip rect is the
full-surface default {0,0,1e9,1e9} in the overwhelmingly common case.

Reserve the high bit of the header flags word (UI_FLAG_CLIP) and have
FillHeader set it only when the clip rect is narrower than the surface
(extracted into the unit-testable UIRenderer::ClipFlags). The shader
gates the compares on the bit; the branch is push-constant-uniform so it
never diverges. When the rect covers the surface the skipped compares
could never reject an in-surface pixel, so output is pixel-identical.

Mirrored on the WebGPU/WGSL path (dom-webgpu.js) for parity. Adds the
UIClipFlag CPU test covering the gate decision.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

implementations/Crafter.Graphics-UI-Shared.cpp

@@ -38,11 +38,27 @@ UIDispatchHeader UIRenderer::FillHeader(std::uint32_t itemBufferSlot,
 #else
     h.frameIdx  = 0;
 #endif
-    h.flags     = flags;
+    h.flags     = ClipFlags(clipRectPx, window_->width, window_->height, flags);
     h._pad      = 0;
     return h;
 }
 
+std::uint32_t UIRenderer::ClipFlags(std::array<float,4> clipRectPx,
+                                    std::uint32_t surfaceWidth,
+                                    std::uint32_t surfaceHeight,
+                                    std::uint32_t flags) noexcept {
+    // The reserved kUIFlagClip bit signals "clip rect is narrower than the
+    // surface". When the rect already covers the whole surface (the common
+    // {0,0,1e9,1e9} default), the four per-pixel clip compares in
+    // uiResolveScreenPixel can never reject an in-surface pixel, so we leave
+    // the bit clear and the shader skips them — pixel-identical output.
+    const bool clipCoversSurface =
+        clipRectPx[0] <= 0.0f && clipRectPx[1] <= 0.0f
+        && clipRectPx[0] + clipRectPx[2] >= static_cast<float>(surfaceWidth)
+        && clipRectPx[1] + clipRectPx[3] >= static_cast<float>(surfaceHeight);
+    return clipCoversSurface ? (flags & ~kUIFlagClip) : (flags | kUIFlagClip);
+}
+
 std::uint32_t UIRenderer::ShapeText(Font& font, float pxSize,
                                     float x, float baselineY,
                                     std::string_view utf8,