feat(testing): per-frame JSONL trace in E2EHarness

Opt-in trace_path parameter dumps one JSON record per processed frame
with the fields diagnostics need:

  frame_idx, timestamp_ns, vo_success, alignment_success,
  tracking_state, confidence,
  eskf_initialized, eskf_position_enu (or None), eskf_pos_sigma_m,
  estimate_lat/lon, gt_lat/lon/alt

No perf cost when trace_path is None. File is rotated per run — safe to
point at /tmp/foo.jsonl for ad-hoc debugging.

First real run on EuRoC MH_01 (100 frames) immediately exposes the
concrete divergence: vo_success=0/100 (VO never engages on EuRoC
grayscale imagery with current SP+LG adapter), eskf_initialized=0/100,
alignment_success=77/100 (satellite-fallback path fires). Diagnosis
that was hidden behind a single "ATE=10.9 km" number is now machine-
readable per frame.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/e2e/test_harness_smoke.py

@@ -64,3 +64,61 @@ async def test_harness_max_frames_larger_than_dataset_is_harmless():
     harness = E2EHarness(adapter, max_frames=100)
     result = await harness.run()
     assert result.num_frames_submitted == 4
+
+
+@pytest.mark.asyncio
+async def test_harness_trace_path_none_does_not_emit(tmp_path):
+    # No trace_path → no file created, no perf cost from serialisation.
+    adapter = SyntheticAdapter(num_frames=3, fps=5.0)
+    harness = E2EHarness(adapter)
+    await harness.run()
+    # Sanity: tmp_path stays empty (harness never touches it).
+    assert list(tmp_path.iterdir()) == []
+
+
+@pytest.mark.asyncio
+async def test_harness_trace_path_writes_jsonl_with_expected_fields(tmp_path):
+    import json
+
+    trace = tmp_path / "run.jsonl"
+    adapter = SyntheticAdapter(num_frames=4, fps=5.0)
+    harness = E2EHarness(adapter, trace_path=trace)
+    await harness.run()
+
+    assert trace.is_file()
+    lines = trace.read_text().splitlines()
+    # One JSON record per submitted frame
+    assert len(lines) == 4
+
+    first = json.loads(lines[0])
+    # Contract: every record carries these keys so downstream tooling can
+    # depend on them across datasets.
+    expected_keys = {
+        "frame_idx",
+        "timestamp_ns",
+        "vo_success",
+        "alignment_success",
+        "tracking_state",
+        "confidence",
+        "eskf_initialized",
+        "eskf_position_enu",     # list[3] or None when not initialized
+        "eskf_pos_sigma_m",      # float or None
+        "estimate_lat",          # float or None
+        "estimate_lon",
+        "gt_lat",
+        "gt_lon",
+        "gt_alt",
+    }
+    missing = expected_keys - set(first.keys())
+    assert not missing, f"trace record missing keys: {missing}"
+
+
+@pytest.mark.asyncio
+async def test_harness_trace_truncates_with_max_frames(tmp_path):
+    # When max_frames caps iteration, trace file has exactly that many lines.
+    trace = tmp_path / "run.jsonl"
+    adapter = SyntheticAdapter(num_frames=10, fps=5.0)
+    harness = E2EHarness(adapter, max_frames=3, trace_path=trace)
+    await harness.run()
+    lines = trace.read_text().splitlines()
+    assert len(lines) == 3