fix(hatch): finite GPU sentinel for boundary loop separators (#386, #416)

Hatch/wipeout boundary sub-loops were separated by NaN vertices that the
fill shaders detected with `p.x == p.x`. Intel Mesa folds that NaN
self-compare to `true` under fast math, so separators read as real
vertices, the ray-cast crossing count broke, and fills bled outside
their boundary — only on Intel iGPUs. GPU uploads now rewrite the
separators to a finite 1e30 sentinel (GPU_BOUNDARY_SEP) and the shaders
test `abs(x) < 1e29`, which every driver evaluates identically. The
CPU-side NaN representation is unchanged.

Co-Authored-By: Claude Fable 5 <[email protected]>

src/shaders/hatch.wgsl

@@ -142,7 +142,11 @@ struct VOut {
 // ── Point-in-polygon (ray casting) over a sub-range of BoundaryBuffer ────
 
 fn valid_vertex(p: vec2<f32>) -> bool {
-    return p.x == p.x && p.y == p.y;
+    // Sub-loop separators are a huge finite sentinel (1e30, see
+    // GPU_BOUNDARY_SEP), not NaN: `x == x` NaN detection gets folded to
+    // `true` by some drivers (Intel Mesa fast math), which made separators
+    // read as real vertices and bleed fills past their boundary (#386, #416).
+    return abs(p.x) < 1.0e29 && abs(p.y) < 1.0e29;
 }
 
 fn edge_crosses(p: vec2<f32>, a: vec2<f32>, c: vec2<f32>) -> bool {

src/shaders/hatch_web.wgsl

@@ -116,7 +116,11 @@ struct VOut {
 // ── Point-in-polygon (ray casting) ────────────────────────────────────────
 
 fn valid_vertex(p: vec2<f32>) -> bool {
-    return p.x == p.x && p.y == p.y;
+    // Sub-loop separators are a huge finite sentinel (1e30, see
+    // GPU_BOUNDARY_SEP), not NaN: `x == x` NaN detection gets folded to
+    // `true` by some drivers (Intel Mesa fast math), which made separators
+    // read as real vertices and bleed fills past their boundary (#386, #416).
+    return abs(p.x) < 1.0e29 && abs(p.y) < 1.0e29;
 }
 
 fn edge_crosses(p: vec2<f32>, a: vec2<f32>, c: vec2<f32>) -> bool {

src/shaders/wipeout.wgsl

@@ -118,7 +118,11 @@ struct VOut {
 // ── Point-in-polygon (ray casting) ────────────────────────────────────────
 
 fn valid_vertex(p: vec2<f32>) -> bool {
-    return p.x == p.x && p.y == p.y;
+    // Sub-loop separators are a huge finite sentinel (1e30, see
+    // GPU_BOUNDARY_SEP), not NaN: `x == x` NaN detection gets folded to
+    // `true` by some drivers (Intel Mesa fast math), which made separators
+    // read as real vertices and bleed fills past their boundary (#386, #416).
+    return abs(p.x) < 1.0e29 && abs(p.y) < 1.0e29;
 }
 
 fn edge_crosses(p: vec2<f32>, a: vec2<f32>, c: vec2<f32>) -> bool {