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": {

content/world/items/coin.json

@@ -1 +0,0 @@
-{ "id": "coin", "name": "coin", "slot": "currency" }

content/world/items/copper.json

@@ -0,0 +1 @@
+{ "id": "copper", "name": "copper coin", "slot": "currency" }

content/world/items/gold.json

@@ -0,0 +1 @@
+{ "id": "gold", "name": "gold coin", "slot": "currency" }

content/world/items/silver.json

@@ -0,0 +1 @@
+{ "id": "silver", "name": "silver coin", "slot": "currency" }