feat(currency): copper/silver/gold replace the coin

The three denominations are move tokens, not stored buckets — there is one
integer, so change-making never arises. The deserter is down to 47c: under a
silver, so he cannot cover a bed and the player sees it on turn one.

A parity test guards the code-side ratios against the content-side ids.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QYa9u7Kdxv5gX4AnwWexy8

content/origins/deserter.json

@@ -15,7 +15,7 @@
   "disposition_overrides": { "brannoc_thane": 40, "cadwyn_vell": 15 },
   "inventory_grants": [
     { "item_id": "worn_shortsword", "qty": 1 },
-    { "item_id": "coin", "qty": 3 }
+    { "item_id": "copper", "qty": 47 }
   ],
   "start_quest_id": "find_the_ledger",
   "build_constraints": {