fix(multiplayer): validate timestamp calendar

Game/scripts/control_plane_client.gd

@@ -305,7 +305,31 @@ static func is_valid_rfc3339_timestamp(value: String) -> bool:
 	if value.is_empty():
 		return false
 	var timestamp_pattern := RegEx.create_from_string("^\\d{4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}(?:\\.\\d+)?(?:Z|[+-]\\d{2}:\\d{2})$")
-	return timestamp_pattern.search(value) != null
+	if timestamp_pattern.search(value) == null:
+		return false
+	var year := int(value.substr(0, 4))
+	var month := int(value.substr(5, 2))
+	var day := int(value.substr(8, 2))
+	var hour := int(value.substr(11, 2))
+	var minute := int(value.substr(14, 2))
+	var second := int(value.substr(17, 2))
+	if month < 1 or month > 12 or hour > 23 or minute > 59 or second > 59:
+		return false
+	var days_in_month := [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
+	var leap_year := year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
+	if leap_year:
+		days_in_month[1] = 29
+	if day < 1 or day > days_in_month[month - 1]:
+		return false
+	var timezone_index := value.find("+", 19)
+	if timezone_index < 0:
+		timezone_index = value.find("-", 19)
+	if timezone_index >= 0:
+		var offset_hour := int(value.substr(timezone_index + 1, 2))
+		var offset_minute := int(value.substr(timezone_index + 4, 2))
+		if offset_hour > 23 or offset_minute > 59:
+			return false
+	return true
 
 
 static func is_valid_resource_id(value: String) -> bool:

Game/tests/cases/test_control_plane_client.gd

@@ -51,6 +51,8 @@ func test_session_expiry_is_checked_at_the_boundary_and_fails_closed() -> void:
 	assert_true(ControlPlaneClient.is_session_expired("1970-01-01T00:16:40Z", 1000), "session expires at the exact boundary")
 	assert_true(ControlPlaneClient.is_session_expired("not-a-timestamp", 1000), "malformed non-empty expiry fails closed")
 	assert_true(ControlPlaneClient.is_valid_rfc3339_timestamp("2026-08-31T12:00:00.123Z"), "fractional RFC3339 timestamp is accepted")
+	assert_true(not ControlPlaneClient.is_valid_rfc3339_timestamp("2026-02-30T12:00:00Z"), "impossible calendar date is rejected")
+	assert_true(not ControlPlaneClient.is_valid_rfc3339_timestamp("2026-13-01T12:00:00Z"), "impossible month is rejected")
 	assert_true(not ControlPlaneClient.is_valid_rfc3339_timestamp("2026-08-31 12:00:00Z"), "space-separated timestamp is rejected")
 	assert_true(not ControlPlaneClient.is_valid_rfc3339_timestamp("2026-08-31T12:00:00"), "timezone-less timestamp is rejected")
 	var valid_session := {"player_id": "player_1234567890", "access_token": "session-id:opaque-token", "expires_at": "2099-08-31T12:00:00Z"}

multiplayer-next.md

@@ -1617,6 +1617,8 @@ The public `/api/v1` route adapters now reject non-opaque queue, proposal, assig
 
 The public queue adapter also rejects an explicitly supplied short or unsafe `ticket_id`; omitted IDs continue to be deterministically server-assigned for idempotent retries.
 
+RFC3339 validation now checks both wire syntax and calendar parseability, rejecting impossible dates before they can become epoch metadata.
+
 Signed MatchNet claims now also require exact JSON string/integer types for every identity, protocol, expiry, slot, team, and generation field; string-number coercion is rejected before canonical signature verification.
 
 Presentation progress: a shared `Game/themes/cosmic_clash_theme.tres` now gives the menu, lobby, matchmaking, and settings surfaces consistent button, input, option, and label styling. The custom-font portion of `TODO.md` remains open until a distributable font asset is selected.