Add supply auto-purchase and preserve save recovery state

scripts/core/simulation_snapshot.gd

@@ -15,16 +15,24 @@ static func capture(clock: SimulationKernel, state: Node, extra := {}) -> Dictio
 
 static func validate(snapshot: Dictionary) -> Dictionary:
 	var errors: Array[String] = []
-	if int(snapshot.get(&"schema_version", -1)) != SCHEMA_VERSION:
+	var schema_value: Variant = snapshot.get(&"schema_version", null)
+	if not _is_nonnegative_integer(schema_value) or int(schema_value) != SCHEMA_VERSION:
 		errors.append("unsupported_schema_version")
 	if not snapshot.get(&"clock", null) is Dictionary:
 		errors.append("missing_clock")
 	else:
 		var clock_data: Dictionary = snapshot[&"clock"]
-		if int(clock_data.get(&"absolute_minute", -1)) < 0:
+		if not _is_nonnegative_integer(clock_data.get(&"absolute_minute", null)):
 			errors.append("invalid_absolute_minute")
-		if int(clock_data.get(&"speed", 0)) not in [1, 2, 3, 4]:
+		var speed_value: Variant = clock_data.get(&"speed", null)
+		if not _is_nonnegative_integer(speed_value) or int(speed_value) not in [1, 2, 3, 4]:
 			errors.append("invalid_speed")
+		# Legacy envelopes may omit these fields and retain restore_clock's
+		# paused=true/version=0 defaults. Present fields cannot be coerced.
+		if typeof(clock_data.get(&"paused", true)) != TYPE_BOOL:
+			errors.append("invalid_paused")
+		if not _is_nonnegative_integer(clock_data.get(&"snapshot_version", 0)):
+			errors.append("invalid_snapshot_version")
 	if not snapshot.get(&"game_state", null) is Dictionary:
 		errors.append("missing_game_state")
 	return {
@@ -32,6 +40,15 @@ static func validate(snapshot: Dictionary) -> Dictionary:
 		&"errors": errors,
 	}
 
+static func _is_nonnegative_integer(value: Variant) -> bool:
+	if typeof(value) == TYPE_INT:
+		return value >= 0
+	# JSON numbers are floats, but exact integral values remain compatible.
+	# Avoid an overflowing float-to-int conversion at the signed int64 limit.
+	if typeof(value) != TYPE_FLOAT or not is_finite(value):
+		return false
+	return value >= 0 and value < 9223372036854775808.0 and value == floor(value)
+
 static func restore(snapshot: Dictionary, clock: SimulationKernel, state: Node) -> bool:
 	var validation := validate(snapshot)
 	if not validation[&"ok"]: