feat(currency): the Currency value type — copper, ratios, formatting

One integer, stored in copper. The ratios are rules and live in code; prices
are content and land in M8. format() suppresses empty denominations so being
broke reads as '47c', not '0g 0s 47c'.

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

client/scripts/state/currency.gd

@@ -0,0 +1,44 @@
+class_name Currency
+extends RefCounted
+## Money. Everything is stored in COPPER — one integer, on GameState.purse_copper.
+## The denominations exist so a bounded §6 move can name a unit (give_item(gold)
+## is +10,000c); they are NOT stored buckets, so change-making never arises.
+##
+## The ratios are rules and live here in code. PRICES are content and live in
+## /content (roadmap: "the economy is two systems"). Formatting happens only at
+## the display edge — this is the one place that knows what "12g 40s 8c" means.
+
+const COPPER := 1
+const SILVER := 100
+const GOLD := 10000
+
+const VALUES := {"copper": COPPER, "silver": SILVER, "gold": GOLD}
+
+
+static func is_currency(item_id: String) -> bool:
+	return VALUES.has(item_id)
+
+
+static func value(item_id: String) -> int:
+	return int(VALUES.get(item_id, 0))
+
+
+static func format(copper: int) -> String:
+	# Empty denominations are suppressed and at least one unit always shows:
+	# "0c", "47c", "1g 8c", "12g 40s". "0g 0s 8c" reads as a spreadsheet, not a
+	# purse — and being broke should read as poverty (§3).
+	if copper <= 0:
+		return "0c"
+	var parts: Array[String] = []
+	var left := copper
+	var gold := left / GOLD          # int division
+	if gold > 0:
+		parts.append("%dg" % gold)
+		left -= gold * GOLD
+	var silver := left / SILVER
+	if silver > 0:
+		parts.append("%ds" % silver)
+		left -= silver * SILVER
+	if left > 0:
+		parts.append("%dc" % left)
+	return " ".join(parts)