fix(creation): pin RNG stream, tighten spend/seed validation

Finding 1 (important): test_same_seed_yields_an_identical_character only
compared two construct() runs against each other, so swapping dex/con in
Attributes.IDS still passed the whole suite while every persisted seed
would silently become a different character. Added a golden-vector test
that pins one fixed seed to its exact rolled attributes and hidden Luck,
so reordering Attributes.IDS or moving the Luck roll fails loudly instead
of silently rewriting every saved character. Proved it: swapped IDS,
watched the golden vector fail, restored, confirmed green.

Also: _validate_spend no longer lets a negative value mask the over-pool
check (continue after each error instead of falling through to `total +=
v`); non-integer spend values (float/bool/string) are now rejected instead
of coerced; a missing or non-integer seed is now a validation error instead
of silently defaulting to 0. GameState's header comment no longer
advertises "stats" it no longer holds.

243 -> 248 tests.

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

client/scripts/newgame/new_game.gd

@@ -111,6 +111,9 @@ static func _validate(origin: Dictionary, world: ContentDB, creation: Dictionary
 	if str(creation.get("name", "")).strip_edges() == "":
 		errors.append("player name is required")
 
+	if not creation.has("seed") or typeof(creation["seed"]) != TYPE_INT:
+		errors.append("seed is required and must be an integer")
+
 	var race_id: String = str(creation.get("race_id", ""))
 	if not Races.exists(race_id):
 		errors.append("unknown race: %s" % race_id)
@@ -138,9 +141,13 @@ static func _validate_spend(spend: Dictionary) -> Array:
 		if not Attributes.exists(str(stat)):
 			errors.append("cannot spend on '%s' — not an attribute" % stat)
 			continue
+		if typeof(spend[stat]) != TYPE_INT:
+			errors.append("spend value for '%s' must be an integer" % stat)
+			continue
 		var v := int(spend[stat])
 		if v < 0:
 			errors.append("spend is additive only: %s is negative" % stat)
+			continue
 		total += v
 	if total > SPEND_POOL:
 		errors.append("spend exceeds the pool of %d (got %d)" % [SPEND_POOL, total])