perf(scene): zoom-adaptive curve tolerance for top-level wires (Phase 3.2)

Edge tessellation used a fixed 0.005 world-unit chord-height tolerance
regardless of zoom — a 1000 m radius arc viewed at 50 mm/px still emitted
hundreds of segments that all collapsed onto the same screen pixel.

`truck_tess` now reads tolerance from a process-wide AtomicU64 override.
`Scene::wires_for_block` sets the override to ~0.5 px chord height
(world_per_pixel × 0.5) before the par_iter, then a Drop guard clears it
back to the default. Floor stays at the original CURVE_TOL so extreme
zoom-in keeps full quality. Block-cache rebuilds and off-render paths
(snap, hit-test) always see the default — the override is bracketed to
the render call only.

Roadmap entry for Phase 3.2 removed; Implementation Order / Key Files
tables updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/mod.rs

@@ -1051,6 +1051,21 @@ impl Scene {
         let blk_ref: &block_cache::BlockCache = &blk_cache;
         let view_aabb = self.view_world_aabb();
         let wpp = self.world_per_pixel();
+        // Zoom-adaptive curve sampling for top-level Edge tessellation. Target
+        // ~0.5 px chord height — far-out arcs that used to emit hundreds of
+        // segments now collapse to a handful. The guard clears the override
+        // when this scope exits so off-render tessellation (snap previews,
+        // hit-test, block_cache rebuild) sees the default.
+        struct CurveTolGuard;
+        impl Drop for CurveTolGuard {
+            fn drop(&mut self) {
+                crate::scene::truck_tess::set_curve_tol_override(None);
+            }
+        }
+        let _tol_guard = wpp.map(|w| {
+            crate::scene::truck_tess::set_curve_tol_override(Some((w * 0.5) as f64));
+            CurveTolGuard
+        });
         let mut wires: Vec<WireModel> = visible
             .into_par_iter()
             .flat_map(|e| {