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

client/tests/unit/test_content_db.gd

@@ -18,7 +18,24 @@ func test_loads_world_ids():
 	assert_true(db.has_location("greywater_docks"))
 	assert_true(db.has_quest("find_the_ledger"))
 	assert_true(db.has_item("worn_shortsword"))
-	assert_true(db.has_item("coin"))
+	assert_true(db.has_item("copper"))
+	assert_true(db.has_item("silver"))
+	assert_true(db.has_item("gold"))
+	assert_false(db.has_item("coin"), "the undifferentiated coin is gone")
+
+
+func test_currency_items_agree_with_the_code():
+	# The denomination ids exist twice — in code (Currency.VALUES, because the
+	# ratios are rules) and in content (slot: "currency"). Guard the two against
+	# drifting apart, since nothing at runtime reconciles them.
+	var content_ids: Array = []
+	for id in db.items:
+		if db.items[id].get("slot", "") == "currency":
+			content_ids.append(id)
+	content_ids.sort()
+	var code_ids: Array = Currency.VALUES.keys()
+	code_ids.sort()
+	assert_eq(content_ids, code_ids)
 
 
 func test_companions_are_brannoc_and_cadwyn():

client/tests/unit/test_new_game.gd

@@ -40,8 +40,9 @@ func test_numeric_luck_absent_from_log():
 
 func test_inventory_and_dispo_land_in_state_not_log():
 	var res := _build({"name": "Aldric", "class_id": "sellsword"})
-	assert_eq(res["state"].inventory.get("coin", 0), 3)
 	assert_eq(res["state"].inventory.get("worn_shortsword", 0), 1)
+	assert_eq(res["state"].purse_copper, 47, "the deserter is down to his last coin")
+	assert_false("copper" in res["state"].inventory, "money is never an inventory row")
 	assert_false("inventory" in res["log"].to_dict())