[AZ-420] Batch 81: FT-P-12 + FT-P-13 GCS scenarios

FT-P-12: parse mavproxy-listener tlog over a 60 s Derkachi replay and
assert SUT->GCS GLOBAL_POSITION_INT cadence lands in [1, 2] Hz (AC-6.1).

FT-P-13: inject `RELOC:<lat>,<lon>,<radius_m>` STATUSTEXT while the SUT
is in dead_reckoned; verify FDR `c8.gcs.operator_command` ack <=2s,
`anchor_search_region` centre shifts toward the hint, and no
BAD_SIGNATURE / UNAUTHORIZED / REJECTED STATUSTEXT lands in the
post-inject window (AC-6.2).

Adds runner.helpers.gcs_telemetry_evaluator (rate, hint-ack correlation,
haversine search-region shift, rejection scan) and
sitl_observer.capture_gcs_tlog (parity surface to capture_ap_tlog).
Pure-logic coverage: 39 new unit tests; full e2e/_unit_tests/ suite
746 passing (was 700). Scenarios skip locally on missing SITL replay
fixture; production hooks (inbound STATUSTEXT parser, anchor_search_region
FDR emitter) tracked outside this task.

See _docs/03_implementation/batch_81_report.md +
reviews/batch_81_review.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

e2e/_unit_tests/helpers/test_sitl_observer.py

@@ -473,6 +473,39 @@ def test_capture_ap_tlog_zero_duration_raises():
         so.capture_ap_tlog(host="x", duration_s=0)
 
 
+# capture_gcs_tlog
+
+
+def test_capture_gcs_tlog_missing_env_raises(unset_replay_dir):
+    # Assert
+    with pytest.raises(RuntimeError, match="env var not set"):
+        so.capture_gcs_tlog(host="sitl-ardupilot", duration_s=1.0)
+
+
+def test_capture_gcs_tlog_missing_file_raises(replay_dir: Path):
+    # Assert
+    with pytest.raises(RuntimeError, match="fixture not found"):
+        so.capture_gcs_tlog(host="sitl-ardupilot", duration_s=1.0)
+
+
+def test_capture_gcs_tlog_returns_path(replay_dir: Path):
+    # Arrange
+    tlog = replay_dir / "gcs_tlog_sitl-ardupilot.tlog"
+    tlog.write_bytes(b"\x00\x01\x02")
+
+    # Act
+    out = so.capture_gcs_tlog(host="sitl-ardupilot", duration_s=1.0)
+
+    # Assert
+    assert out == tlog
+
+
+def test_capture_gcs_tlog_zero_duration_raises():
+    # Assert
+    with pytest.raises(RuntimeError, match="duration_s must be positive"):
+        so.capture_gcs_tlog(host="x", duration_s=0)
+
+
 # read_ap_parameter