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/app/history.rs

@@ -978,6 +978,9 @@ impl OpenCADStudio {
         }
         let changes = self.tabs[i].scene.apply_entity_delta(&d.entities, undo);
         let scene = &mut self.tabs[i].scene;
+        if !d.parametric_constraints.is_empty() {
+            scene.bump_constraints_epoch();
+        }
         for entry in &d.parametric_constraints {
             let value = if undo { &entry.before } else { &entry.after };
             *scene.parametric_constraint_set_mut(entry.scope) = value.clone();