Persistent capture window over a fixed sub-rectangle of the captured surface. open() allocates a stable buffer once; sync() re-fills it all-or-nothing from the current latest frame under one seqlock window (byte-exact, never torn). Pull only - no poll/thread inside; the caller drives. FLAT/ROWS layouts and BGRA/RGB24 formats chosen at open, orthogonal. Strict skip when the rect does not fit the current frame; no frame_id dedup.
Extract read_field into perception-internal.h as vgpup_read_field to share the producer-field read between sample.c and capture.c. Bump version to 0.3.14.
