fix: visually block off-hand slot when 2H weapon equipped in main hand

- Add getTwoHandedBlocker() helper to EquipmentSlotGrid that detects when
  mainHand has a two-handed weapon and returns the weapon name
- Render offHand slot with Lock icon + 'Blocked: <weapon name>' label +
  reduced opacity when blocked, distinct from normal 'Empty' dashed-border slot
- Add regression test verifying all 4 two-handed types are correctly flagged
  and non-two-handed types remain unblocked (11 tests)

docs/dependency-graph.json

@@ -1,6 +1,6 @@
 {
   "_meta": {
-    "generated": "2026-06-09T12:49:06.615Z",
+    "generated": "2026-06-09T13:31:28.048Z",
     "description": "Import dependency graph for src/lib/game. Keys are files, values are arrays of files they import.",
     "usage": "To find what a file affects, search for its path in the VALUES. To find what a file depends on, look at its KEY entry."
   },