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/view/render.rs

@@ -1202,7 +1202,7 @@ impl Scene {
             // actively moving; the scene-render cache holds the full-quality
             // (hatched) frame once it settles. Only applied to the on-screen
             // Model / paper content — the paper *sheet* keeps its fills.
-            skip_hatch: !inst.paper_sheet && self.navigating_lod(),
+            skip_hatch: self.hatch_lod_enabled() && !inst.paper_sheet && self.navigating_lod(),
             geometry_epoch: self.geometry_epoch,
             camera_generation: self.camera_generation,
             wire_content_id,