perf(hatch): tessellate fills so navigation no longer flickers (#258)

The batched hatch pass drew a screen-covering AABB quad per hatch and,
for every fragment, ran a point-in-polygon ray-cast over the whole
boundary plus the pattern test, with a discard that disabled early-Z.
Zoomed into a hatch the quad covers the screen, so the shader re-ran
per pixel every frame — the cost the PR #273 interaction-LOD worked
around by suppressing hatches during navigation, which made fills
flicker on and off while zooming.

Triangulate each boundary once (lyon, even-odd — same coverage as the
old in_polygon, islands/holes included) and rasterize those triangles
instead: no screen-filling overdraw, no per-fragment boundary test. The
fragment shader keeps the boundary test only on a per-instance fallback
path (poly_test == 1) for boundaries that fail to tessellate.
Tessellation is cached with the batch build (geometry_epoch) in local
space, so the double-single relative-to-eye precision is preserved.

With the fast path the interaction-LOD is no longer needed; it is gated
behind OCS_HATCH_LOD (default off) as a safety net for now.

src/scene/mod.rs

@@ -1198,6 +1198,16 @@ impl Scene {
             .map_or(false, |t| t.elapsed().as_millis() < Self::NAV_SETTLE_MS)
     }
 
+    /// Whether the interaction-LOD hatch suppression is enabled (env
+    /// `OCS_HATCH_LOD`), read once. Default OFF: the tessellated hatch pass is
+    /// cheap enough that suppression — and its zoom flicker (#258) — is not
+    /// needed. Kept behind the flag as a safety net for pathological drawings.
+    pub fn hatch_lod_enabled(&self) -> bool {
+        use std::sync::OnceLock;
+        static ON: OnceLock<bool> = OnceLock::new();
+        *ON.get_or_init(|| std::env::var_os("OCS_HATCH_LOD").is_some())
+    }
+
     /// True for a short window around navigation — used by the subscription to
     /// keep requesting frames just past the settle point so the one full-quality
     /// (hatched) frame actually renders after the cursor stops, even when no