sema: fix host stack-overflow hang; robust VA resolution; dispatcher lock fix

- svc_handle_chan_dump()/svc_decode_stream(): static snapshot buffers. The
  128-channel local array made an ~18 KB frame, which overflows the 16 KB
  kernel stack in the function prologue and panicked the host 2-3 s after a
  VM attached the vGPU (proven from EFI pstore: guard page hit at
  svc_handle_chan_dump+0x20, sub rsp,0x4850).
- svc_pump_channels(): write-back now also persists expected_sem_payload,
  rescue_last and rescue_fail. The pump works on a snapshot copy, so
  without this the stale-entry skip and the rescue rate limit could never
  fire and the payload expectation was reinitialized every pass.
- svc_targeted_resolve(): sliced scan (128 MB per call, one VA at a time,
  failed-VA cache) instead of a full-RAM scan per miss; no longer appends
  to the sorted map out of order.
- svc_rebuild(): merges new scan results into the previous map (fresh
  translation wins in place, new VAs appended) instead of replacing it
  wholesale, so live translations survive a scan-yield collapse.
- Warning fixes: misleading indentation, unused variable, jiffies
  typecheck. Frame audit: largest frame now 872 B.
- Includes the previous session's uncommitted work: sema_counter /
  shadow_scan / shadow_dump debug params, exit stage markers, pdev ref
  fix, and the dynamic dispatcher request_firmware-out-of-spinlock fix.

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