vgsp: m171 event completion fixes (minimal cudaEventSynchronize repro now passes) + AGENT-F R2 write/read consistency ledger

- gsp_rm_emulation: the event burst budget becomes a time-windowed rate
  (1 s) with completion posts allowed headroom over hint re-posts; the
  periodic hint dedups events posted within 250 ms (strictly below its own
  500 ms cadence - an equal window races the boundary and skips forever);
  force_post skips retired slots (rc=22 spam was an artifact). Verified:
  the minimal cudaEventCreate/Record/Synchronize repro and a second repro
  with cross-stream waits and elapsed-time now complete on a live boot;
  previously cudaEventSynchronize hung forever.
- gsp_sema_service: write/read consistency ledger (AGENT-F R2). Large
  device-destination writes record {va_page -> pa, aperture}; the launch
  data fill and device-source reads prefer the recorded page whenever
  their own resolution disagrees (sema-ledger DIVERGE log). Session
  scoped, zero cost while empty.

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