feat(client): id helper + leaf canon-log entities with owned invariants

Also configure GUT to not treat push_error() as a test failure
(client/.gutconfig.json: failure_error_types = [engine, gut]).
The brief's LogPlayer/Quest setters intentionally push_error() on
rejected input; GUT's default failure_error_types includes
push_error, which would fail test_player_rejects_unknown_class and
test_quest_status_enum despite their assertions passing.

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

client/scripts/ids.gd

@@ -0,0 +1,9 @@
+class_name Ids
+extends RefCounted
+## Shared id validation. Every canon-log / content id matches ^[a-z0-9_]+$.
+
+static var _re: RegEx = RegEx.create_from_string("^[a-z0-9_]+$")
+
+
+static func is_valid(id: String) -> bool:
+	return _re.search(id) != null