feat(M01): add typed domain identities

Work-Package: M01-FND-DOMAIN-IDENTITIES-001

Agent: sindo-main-codex

src/domain/identity/entity_id.gd

@@ -0,0 +1,31 @@
+class_name EntityId
+extends RefCounted
+
+## Session-local runtime entity identity.
+## This sequence is never a stable content ID, server entry or WoW wire GUID.
+
+var session_sequence: int:
+	get:
+		return _session_sequence
+
+var _session_sequence: int
+
+
+## Stores an allocator-issued session sequence without persistence semantics.
+func _init(session_sequence_value: int) -> void:
+	_session_sequence = session_sequence_value
+
+
+## Reports whether this is a positive allocator-issued sequence.
+func is_valid() -> bool:
+	return _session_sequence > 0
+
+
+## Compares identity inside the same explicitly owned runtime session.
+func equals(other_entity_id: EntityId) -> bool:
+	return other_entity_id != null and _session_sequence == other_entity_id.session_sequence
+
+
+## Returns a diagnostic key; it must not be persisted across sessions.
+func to_debug_key() -> String:
+	return "entity:%d" % _session_sequence