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.

benches/README.md

@@ -19,7 +19,7 @@ cargo bench --bench performance_benchmarks -- --quick
 
 ---
 
-## Benchmark Suite Catalog (29 Metrics)
+## Benchmark Suite Catalog (30 Metrics)
 
 ### 1. Scene & Entity Management
 - `scene_entity_ingestion_10k`: Ingestion throughput for 10,000 mixed 2D entities (Lines, Circles, Arcs, Polylines).
@@ -55,6 +55,7 @@ cargo bench --bench performance_benchmarks -- --quick
 - `ui_statusbar_derived_data`: Status-bar cached derived-data hit (layout names + scale picker `Arc` clones).
 - `ui_grip_budget`: Budgeted per-frame selection-grip projection (dense-polyline grips capped to `MAX_SELECTED_GRIPS`).
 - `ui_grip_budget_build`: Selection-change `apply_grip_budget` cost over the full grip set (sort + FxHashSet + filter).
+- `ui_constraint_glyphs`: Constraint-glyph cache hit (`cached_glyph_placements` key build + `Arc` clone over dozens of glyphs, no recompute).
 
 ### 6. State Management & Draworder
 - `selection_state_deep_clone`: Deep cloning of `SelectionState` (with 64 polygon points).