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>

src/backend/api/routes.py

@@ -245,10 +245,12 @@ def _next_frame_id() -> int:
 
 
 def _frame_to_dict(frame: CameraFrame, include_images: bool = True) -> dict:
-    """/api/capture and /ws JSON of a frame. frame_id counts responses, as before."""
+    """/api/capture and /ws JSON of a frame. frame_id counts responses, as before;
+    frameset_seq (added field) numbers the camera frameset, as /api/raw/latest does."""
     enc = encoded(frame)
     result = {
         "frame_id": _next_frame_id(),
+        "frameset_seq": frame.frameset_seq,
         "timestamp": frame.timestamp,
         "rgb_shape": list(frame.rgb.shape),
         "depth_shape": list(frame.depth.shape),