feat(currency): purse_copper on GameState, with grant/take routing

One int holds all money. grant/take are the sole routing point — currency ids
go to the purse, everything else to inventory. Money is never an inventory row.

The 'coin' stand-in in the two unrelated inventory tests becomes
worn_shortsword; they were never about money.

client/scripts/state/game_state.gd

@@ -8,7 +8,8 @@ var luck: int = 0
 var luck_base: int = 0
 var stats: Dictionary = {}            # {str, dex, con, fth, mag}
 var npc_dispositions: Dictionary = {} # world-npc id -> int (−100..100)
-var inventory: Dictionary = {}        # item_id -> qty
+var inventory: Dictionary = {}        # item_id -> qty (NEVER money — see purse_copper)
+var purse_copper: int = 0             # ALL money, in copper. One integer (§4.1 of the spec).
 var revealed_topics: Dictionary = {}  # topic_id -> true
 var gifts_given: Dictionary = {}      # item_id -> true (idempotency for give_item)
 
@@ -41,6 +42,27 @@ func remove_item(item_id: String, qty: int) -> void:
 		inventory.erase(item_id)
 
 
+func add_copper(amount: int) -> void:
+	purse_copper = maxi(0, purse_copper + amount)
+
+
+func grant(item_id: String, qty: int) -> void:
+	# The single routing point (with take): currency lands in the purse, every
+	# other item in the inventory dict. Money is never an inventory row, so it
+	# can never surface in an inventory grid — exclusion by construction.
+	if Currency.is_currency(item_id):
+		add_copper(Currency.value(item_id) * qty)
+	else:
+		add_item(item_id, qty)
+
+
+func take(item_id: String, qty: int) -> void:
+	if Currency.is_currency(item_id):
+		add_copper(-Currency.value(item_id) * qty)
+	else:
+		remove_item(item_id, qty)
+
+
 func mark_revealed(topic_id: String) -> void:
 	revealed_topics[topic_id] = true