perf(glyphs): memoize constraint-glyph placements behind constraints_epoch

Placements (entity lookups, intersections, String labels, projections)
recomputed per view() and per mouse-move hit-test, with overlay re-clones
and 2-3x offset recomputes. New constraints_epoch bumped on every set
mutation/visibility/undo/DWG-load (plus new set_constraint_enabled API);
single-slot Arc<[GlyphEntry]> memo on an 11-field key (hover/selection
excluded); overlay de-cloned/de-duped to the single geometry source;
hit paths reuse the cache.

Tests:
- cached_glyph_placements_hit_returns_same_arc: same-key Arc identity
- cached_glyph_placements_invalidated_by_metadata_edit: edit + selection change yield new Arc
- glyph_hit_test_entries_matches_precomputed_boxes: shared hit-test helper

Bench (cargo bench glyph, 60 constraints): 8.18us -> 0.05-0.15us (~55x).
A/B reference deleted.

src/scene/dwg_native_constraints.rs

@@ -2841,6 +2841,10 @@ impl Scene {
                 self.parametric_constraints.push(set);
             }
         }
+        // Whole-set replace (file open / reload): covers the DWG-import
+        // `enabled` / `axis_direction` / `distance_direction` / `angle_sector`
+        // writes, which happen on the local set during decode.
+        self.bump_constraints_epoch();
     }
 
     /// Makes the standard associative graph match the live command model.