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)

client/scripts/state/currency.gd.uid

@@ -0,0 +1 @@
+uid://cwnflbwacp5jf

client/tests/unit/test_currency.gd

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+extends "res://addons/gut/test.gd"
+
+
+func test_format_zero_is_copper():
+	assert_eq(Currency.format(0), "0c")
+
+
+func test_format_sub_silver():
+	assert_eq(Currency.format(8), "8c")
+	assert_eq(Currency.format(47), "47c")
+
+
+func test_format_exact_denominations():
+	assert_eq(Currency.format(100), "1s")
+	assert_eq(Currency.format(10000), "1g")
+
+
+func test_format_full_ladder():
+	assert_eq(Currency.format(143), "1s 43c")
+	assert_eq(Currency.format(347), "3s 47c")
+	assert_eq(Currency.format(124008), "12g 40s 8c")
+
+
+func test_format_skips_empty_middle_and_tail():
+	assert_eq(Currency.format(10008), "1g 8c", "an empty silver place is skipped, not zero-padded")
+	assert_eq(Currency.format(124000), "12g 40s", "an empty copper place is skipped")
+
+
+func test_value_of_each_denomination():
+	assert_eq(Currency.value("copper"), 1)
+	assert_eq(Currency.value("silver"), 100)
+	assert_eq(Currency.value("gold"), 10000)
+
+
+func test_non_currency_id_is_not_money():
+	assert_false(Currency.is_currency("worn_shortsword"))
+	assert_eq(Currency.value("worn_shortsword"), 0)
+
+
+func test_currency_ids_are_currency():
+	for id in ["copper", "silver", "gold"]:
+		assert_true(Currency.is_currency(id), "%s is money" % id)

client/tests/unit/test_currency.gd.uid

@@ -0,0 +1 @@
+uid://dwpdm680q83ws