feat: frameset_seq in /api/capture and /ws, added field

The number of the camera frameset an aligned frame comes from, in the same
numbering as /api/raw/latest and /api/color/latest: it lets a tool compare
Intel's align on PC2 with an align on the host on the SAME frameset.
Existing fields unchanged (frame_id still counts responses); the field-set
tests now say "the R01 fields plus frameset_seq only".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_parallel_capture.py

@@ -18,11 +18,13 @@ from src.backend.main import app  # noqa: E402
 from src.backend.services.camera import RealSenseD435  # noqa: E402
 from tests.fake_realsense import FakePipeline, open_fake_camera  # noqa: E402
 
-# Fields of /api/capture before optimize-streams R01 (the multi-camera service parses them).
+# Fields of /api/capture before optimize-streams R01 (the multi-camera service parses them),
+# and the only one added since (R05): existing fields never change.
 CAPTURE_FIELDS = {
     "frame_id", "timestamp", "rgb_shape", "depth_shape", "depth_min_mm", "depth_max_mm",
     "depth_valid_pixels", "rgb_jpeg_b64", "depth_png_b64", "depth_color_b64",
 }
+ADDED_FIELDS = {"frameset_seq"}
 
 
 @pytest.fixture
@@ -67,7 +69,7 @@ def test_capture_fields_unchanged_and_one_frame_shared(fake_camera):
     bodies = asyncio.run(run())
     assert all(b["success"] for b in bodies)
     data = [b["data"] for b in bodies]
-    assert all(set(d) == CAPTURE_FIELDS for d in data)
+    assert all(set(d) == CAPTURE_FIELDS | ADDED_FIELDS for d in data)
     assert len({d["frame_id"] for d in data}) == 3            # frame_id still counts responses
     assert len({d["timestamp"] for d in data}) == 1           # ... all three served by one frame
     assert len({d["rgb_jpeg_b64"] for d in data}) == 1        # ... with the same bytes
@@ -89,7 +91,7 @@ def test_websocket_streams_frames(fake_camera):
         ws.send_text("start")
         first, second = ws.receive_json(), ws.receive_json()
         ws.send_text("close")
-    assert set(first) == CAPTURE_FIELDS
+    assert set(first) == CAPTURE_FIELDS | ADDED_FIELDS
     assert second["timestamp"] > first["timestamp"]