feat(client): Luck (deterministic gen/drift/descriptor) + GameState store

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/scripts/state/luck.gd

@@ -0,0 +1,35 @@
+class_name Luck
+extends RefCounted
+## Numeric Luck (charter §7). Lives in GameState; only descriptor() reaches the
+## log. Deterministic given a seeded RNG (charter §10). Die/range/bands are
+## TUNABLE constants — not final values.
+
+const MIN := 1
+const MAX := 20
+const DRIFT := 3
+
+const BANDS := [
+	{"max": 4, "text": "Fortune spits on you"},
+	{"max": 8, "text": "The world is not on your side"},
+	{"max": 12, "text": "The dice are indifferent"},
+	{"max": 16, "text": "Luck rides with you"},
+	{"max": 20, "text": "The dice are kind today"},
+]
+
+
+static func roll_base(rng: RandomNumberGenerator, modifier: int) -> int:
+	var total := 0
+	for i in 5:
+		total += rng.randi_range(1, 20)
+	return clampi(roundi(total / 5.0) + modifier, MIN, MAX)
+
+
+static func drift(current: int, base: int, delta: int) -> int:
+	return clampi(clampi(current + delta, base - DRIFT, base + DRIFT), MIN, MAX)
+
+
+static func descriptor(luck: int) -> String:
+	for band in BANDS:
+		if luck <= band["max"]:
+			return band["text"]
+	return BANDS[-1]["text"]