vgsp: retry deferred display completions; escalate the UVM init-slot release

Two live walls from the r1649/r1650 windowed-GZDoom runs:

1. Display completion wake-up. All NV01_EVENT registrations carry
   hSrcResource=0, so gsp_rm_post_channel_event() always takes the shotgun
   fallback, which skips events posted within the 250 ms hint dedup. When the
   display UPDATE is the channel's last push, the pump's retry was gated on
   new work (put != consumed) and never ran, so the X server's flip wait
   spun in _nv018351rm/os_schedule forever. Add
   gsp_rm_post_channel_event_force() (bypasses only the hint dedup; the
   event burst cap and status-ring free-space discipline still apply), a
   per-channel event_retry_at, and an idle-pass retry every 250 ms that
   logs "re-posted after deferral". Also move event_burst_at after the cap
   check so refused retries cannot keep the window fresh.

2. UVM channel init slot. r1651 gdb proved the channel's tracker entry waits
   for value 2 while the authored pool word reads 1; a manual write of 2
   cleared uvm_push_end_and_wait. The channel's control-entry + init pushes
   are boot-time configuration, so keep the rank slot moving for 10 s after
   first submission: one attempt per 250 ms per rank, escalating 1->2->3->4
   through every published VAS translation. Values beyond that need the
   real CE push decode.

Evidence: backups/evidence-20261002-victory/r1649/ (X spin perf, event
journal), r1650/ (re-post lines), r1651/ (tracker entry value=2, manual
write clears the spin).

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