vgsp: fail4 r297-r300 - decode-time staging snapshot (H2D shapes only) + ops-order release guard

1. CONT.53 step (1): device-dst copies from host sources now snapshot the
   source at op-build (bounded pool 16 x 4 MB, 30 s TTL) and execute
   from the snapshot, so the staging read happens at the push's
   processing position.
2. The capture gate is aperture-based (dst VID, src SYS) after the r300
   captures showed a naive VA-floor gate also caught D2H readbacks and
   froze pre-kernel zeros.
3. Ops-order release guard: a pass that built ops queues its releases
   through the pending pass instead of applying inline, so a fence
   never clears before the pass's copy ops read their source.
A/B (r297/r299/r300): snapshots fire (6-7 per run) but the +2 MB phase
persists (win@0/1M = exp[+2M]); the r300 capture contents show the H2D
op sources already carry late-chunk data at decode - the scrambling is
at the driver's staging hand-off into the push stream, before our
service sees the op (next instrument: correlate the client's PTRS with
per-capture buffer identity).

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