feat(sessions): add durable observation archive and replay API

tests/test_viewer_replay.py

@@ -16,6 +16,14 @@ def _write_native(path: Path, topic: str, payload: bytes) -> None:
     )
 
 
+def _append_native(path: Path, topic: str, payload: bytes) -> None:
+    topic_raw = topic.encode()
+    with path.open("ab") as stream:
+        stream.write(FRAME_HEADER.pack(len(topic_raw), len(payload)))
+        stream.write(topic_raw)
+        stream.write(payload)
+
+
 def test_native_replay_uses_aligned_metadata_timing(tmp_path: Path) -> None:
     capture = tmp_path / "mqtt.raw.k1mqtt"
     _write_native(capture, "lixel/application/report/lio_pose", b"pose")
@@ -39,6 +47,35 @@ def test_native_replay_uses_aligned_metadata_timing(tmp_path: Path) -> None:
     assert message.received_monotonic_ns == 456
 
 
+def test_native_replay_stops_at_crash_truncated_metadata_tail(tmp_path: Path) -> None:
+    capture = tmp_path / "mqtt.raw.k1mqtt"
+    _write_native(capture, "RealtimePointcloud", b"first")
+    _append_native(capture, "RealtimePointcloud", b"uncommitted-tail")
+    first_record = {
+        "record_type": "message",
+        "sequence": 1,
+        "received_at_epoch_ns": 123_000_000_000,
+        "received_monotonic_ns": 456,
+    }
+    (tmp_path / "mqtt.metadata.jsonl").write_text(
+        json.dumps(first_record) + "\n" + '{"record_type":"message"',
+        encoding="utf-8",
+    )
+
+    messages = list(iter_replay_messages(capture))
+
+    assert [message.payload for message in messages] == [b"first"]
+
+
+def test_native_replay_rejects_newline_terminated_metadata_corruption(tmp_path: Path) -> None:
+    capture = tmp_path / "mqtt.raw.k1mqtt"
+    _write_native(capture, "RealtimePointcloud", b"frame")
+    (tmp_path / "mqtt.metadata.jsonl").write_text("{corrupt\n", encoding="utf-8")
+
+    with pytest.raises(ReplayFormatError, match="not valid JSON"):
+        list(iter_replay_messages(capture))
+
+
 def test_legacy_tsv_replay_validates_and_decodes_payload(tmp_path: Path) -> None:
     capture = tmp_path / "payloads.tsv"
     capture.write_bytes(b"1784124315.186225000\tRealtimePath\t4\t0001aaff\n")