perf(lidar): avoid duplicate replay decode during build

tests/test_lidar_replay.py

@@ -10,6 +10,7 @@ import pytest
 from fastapi import APIRouter
 from fastapi.routing import APIRoute
 
+import k1link.compute.lidar_replay as lidar_replay_module
 from k1link.compute import (
     K1_LIDAR_PACK_V2_PROFILE,
     K1LocalSurfaceV1,
@@ -253,6 +254,35 @@ def test_lidar_replay_v2_retains_fields_and_passes_equivalence(tmp_path: Path) -
     assert build_lidar_replay_pack_v2(capture, tmp_path / "packs") == output
 
 
+def test_lidar_replay_build_decodes_source_once(
+    tmp_path: Path,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    capture = _capture(tmp_path)
+    original_capture_arrays = lidar_replay_module._capture_arrays
+    calls: list[Path] = []
+
+    def counting_capture_arrays(path: Path) -> dict[str, Any]:
+        calls.append(path)
+        return original_capture_arrays(path)
+
+    monkeypatch.setattr(
+        lidar_replay_module,
+        "_capture_arrays",
+        counting_capture_arrays,
+    )
+
+    output = build_lidar_replay_pack_v2(capture, tmp_path / "packs")
+    pack = LidarReplayPackV2(output)
+    try:
+        assert pack.equivalence["status"] == "passed"
+        assert pack.equivalence["array_mismatches"] == 0
+    finally:
+        pack.close()
+
+    assert calls == [capture.resolve(strict=True)]
+
+
 def test_lidar_replay_v2_unblocks_detector_input_but_not_mapping() -> None:
     assessments = {
         item.stage: item for item in assess_lidar_profile(K1_LIDAR_PACK_V2_PROFILE)