Add filename identity validator fixture

Author: Wonderforge-Lab

tests/fixtures/invalid/filename/not-the-packet-id.json

@@ -0,0 +1,21 @@
+{
+  "record_type": "packet",
+  "schema_version": 1,
+  "packet_id": "20260902-example-session-mismatch",
+  "created_at": "2026-09-02T12:00:00Z",
+  "created_by": "example-session",
+  "deposited_by": "example-session",
+  "content_origin": "operator_authored",
+  "source_refs": [],
+  "derivative_of": [],
+  "provenance_coverage": "record_level",
+  "source_session": "example-session",
+  "target_session": "review-session",
+  "topic": "mismatch",
+  "status": "new",
+  "path": "tests/fixtures/valid/packet/artifact.md",
+  "response_expected": false,
+  "response_packet_id": null,
+  "tags": [],
+  "notes": "Invalid: canonical filename does not match packet_id."
+}