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_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())