fix(hatch): delay complex pattern LOD

Keep every pattern family visible until all projected spacings
become sub-pixel, then switch the hatch to solid.

Refs #569

src/shaders/hatch.wgsl

@@ -319,27 +319,31 @@ fn check_family(
         return mix(inst.color2, inst.color, t);
     }
 
-    // 3. Pattern LOD: when the densest family's spacing projects below
-    //    2 px, lines blur into a solid fill — return color instead of
-    //    iterating every family (mirrors Phase 3.3 LOD in hatch.wgsl).
-    if u.world_per_pixel > 0.0 && inst.family_count > 0u {
-        var min_spacing_world: f32 = 1.0e30;
+    // 3. Pattern LOD. Keep every family visible until all family spacings
+    //    project below 2 px, then substitute one solid fill. A single dense
+    //    family must neither hide itself nor turn a complex hatch solid.
+    var all_families_subpixel = inst.family_count > 0u && u.world_per_pixel > 0.0;
+    if all_families_subpixel {
         for (var i = 0u; i < inst.family_count; i++) {
-            let s = abs(families[inst.family_offset + i].perp_step) * inst.scale;
-            if s > 0.0 && s < min_spacing_world {
-                min_spacing_world = s;
+            let spacing_world =
+                abs(families[inst.family_offset + i].perp_step) * inst.scale;
+            if spacing_world <= 0.0 {
+                all_families_subpixel = false;
+            } else if spacing_world / u.world_per_pixel >= 2.0 {
+                all_families_subpixel = false;
             }
         }
-        if min_spacing_world / u.world_per_pixel < 2.0 {
-            return inst.color;
-        }
+    }
+    if all_families_subpixel {
+        return inst.color;
     }
 
     // 4. Pattern evaluation.
     let cos_off = cos(inst.angle_offset);
     let sin_off = sin(inst.angle_offset);
     for (var i = 0u; i < inst.family_count; i++) {
-        if check_family(v.xz, ddx_xz, ddy_xz, families[inst.family_offset + i], cos_off, sin_off, inst.scale) {
+        let fam = families[inst.family_offset + i];
+        if check_family(v.xz, ddx_xz, ddy_xz, fam, cos_off, sin_off, inst.scale) {
             return inst.color;
         }
     }

src/shaders/hatch_texture.wgsl

@@ -272,25 +272,29 @@ fn check_family(
         return mix(h.color2, h.color, t);
     }
 
-    // Pattern LOD: when the densest family projects below 2 px, lines blur into
-    // a solid fill — return solid instead of looping every family.
-    if u.world_per_pixel > 0.0 && h.n_families > 0u {
-        var min_spacing_world: f32 = 1.0e30;
+    // Keep every family visible until all family spacings project below 2 px,
+    // then substitute one solid fill. A single dense family must neither hide
+    // itself nor turn a complex hatch solid.
+    var all_families_subpixel = h.n_families > 0u && u.world_per_pixel > 0.0;
+    if all_families_subpixel {
         for (var i = 0u; i < h.n_families; i++) {
-            let s = abs(load_family(i).perp_step) * h.scale;
-            if s > 0.0 && s < min_spacing_world {
-                min_spacing_world = s;
+            let spacing_world = abs(load_family(i).perp_step) * h.scale;
+            if spacing_world <= 0.0 {
+                all_families_subpixel = false;
+            } else if spacing_world / u.world_per_pixel >= 2.0 {
+                all_families_subpixel = false;
             }
         }
-        if min_spacing_world / u.world_per_pixel < 2.0 {
-            return h.color;
-        }
+    }
+    if all_families_subpixel {
+        return h.color;
     }
 
     let cos_off = cos(h.angle_offset);
     let sin_off = sin(h.angle_offset);
     for (var i = 0u; i < h.n_families; i++) {
-        if check_family(v.xz, ddx_xz, ddy_xz, load_family(i), cos_off, sin_off, h.scale) {
+        let fam = load_family(i);
+        if check_family(v.xz, ddx_xz, ddy_xz, fam, cos_off, sin_off, h.scale) {
             return h.color;
         }
     }

tests/hatch_shader_lod.rs

@@ -0,0 +1,40 @@
+const STORAGE_SHADER: &str = include_str!("../src/shaders/hatch.wgsl");
+const TEXTURE_SHADER: &str = include_str!("../src/shaders/hatch_texture.wgsl");
+
+fn assert_all_family_lod(source: &str, name: &str) {
+    let module = naga::front::wgsl::parse_str(source)
+        .unwrap_or_else(|error| panic!("{name} WGSL must parse: {error}"));
+    naga::valid::Validator::new(
+        naga::valid::ValidationFlags::all(),
+        naga::valid::Capabilities::empty(),
+    )
+    .validate(&module)
+    .unwrap_or_else(|error| panic!("{name} WGSL must validate: {error}"));
+
+    assert!(
+        source.contains("var all_families_subpixel"),
+        "{name} must track the all-family LOD transition"
+    );
+    assert!(
+        source.contains("if all_families_subpixel"),
+        "{name} must solid-fill only at the all-family transition"
+    );
+    assert!(
+        !source.contains("if is_subpixel") && !source.contains("visible_families"),
+        "{name} must not skip individual families before LOD activates"
+    );
+    assert!(
+        !source.contains("min_spacing_world"),
+        "{name} must not solid-fill a complex pattern because of one dense family"
+    );
+}
+
+#[test]
+fn storage_hatch_lod_waits_for_every_family() {
+    assert_all_family_lod(STORAGE_SHADER, "storage hatch");
+}
+
+#[test]
+fn texture_hatch_lod_waits_for_every_family() {
+    assert_all_family_lod(TEXTURE_SHADER, "texture hatch");
+}