fix(block-cache): harden fill_tris_low access against unpaired wires

emit_wire reconstructed fill verts via a raw `fill_tris_low[idx]` index,
guarded only by a debug_assert. Slice bounds checks are not gated by
debug-assertions, so a Leader with a filled arrowhead nested in a block
(populated fill_tris, empty fill_tris_low via the low-level tessellate
early-return) panicked in release as well as debug.

Use the panic-safe `.get(idx).copied().unwrap_or([0.0; 3])` — the same
"empty = all-zero low half" contract the other fill consumers
(face3d_gpu, xclip) already honour. The debug_assert stays as a tripwire.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/cache/block_cache.rs

@@ -1413,7 +1413,13 @@ fn emit_wire(
     }
     debug_assert_eq!(lw.fill_tris.len(), lw.fill_tris_low.len());
     for (idx, p) in lw.fill_tris.iter().enumerate() {
-        let pl = lw.fill_tris_low[idx];
+        // Empty/short fill_tris_low means "no low half" (all-zero), per the
+        // WireModel contract — same panic-safe access the other fill consumers
+        // use (face3d_gpu, xclip). A Leader with a filled arrowhead nested in a
+        // block reaches here with populated fill_tris but empty fill_tris_low;
+        // a raw `[idx]` would panic in release (bounds checks are not gated by
+        // debug-assertions). The debug_assert above stays as a tripwire.
+        let pl = lw.fill_tris_low.get(idx).copied().unwrap_or([0.0; 3]);
         let v = accum_xform.apply(Vector3::new(
             p[0] as f64 + pl[0] as f64,
             p[1] as f64 + pl[1] as f64,