FrameAcquisition: a single thread calls wait_for_frames + align (the old
capture() body, now _grab_once) and keeps the latest frame, numbered.
capture() no longer touches the pipeline: it waits for the next frame of
that thread; requests waiting at the same moment get the same frame, so N
callers cost one grab. get_rgb()/get_depth() keep their 100 ms cache,
now on the thread's latest frame (_last_frame is gone).
The thread grabs on demand, only while someone waits for a newer frame:
grabbing continuously at 30 fps would align frames nobody reads, and align
on the Jetson CPU is the main suspect of the capture cost. A grab error is
logged and retried, the thread never dies. close() stops it before the
pipeline and joins it after (pipeline.stop() ends a wait in progress).
open/close and camera_open are unchanged, so the healthcheck + autoheal
recovery of ede70a2 sees the same state as before.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
