kernel: track channels and execute host-class pushed work

The guest's CUDA stack was stuck inside context creation because the work it
submits is never executed by the emulated GPU. Two forms of pushed work were
missing, and the completion path had grown far too slow to be useful:

- Channel tracking: on a channel allocation parse the GPFIFO GPU VA and entry
  count, the USERD address/aperture and the instance block, hand them to the
  progress service and drop them again when the channel is freed.

- Host-class (C46F) semaphore releases: pushes on a compute channel signal
  progress with a five-dword method run (SEM_ADDR_LO/HI, PAYLOAD_LO/HI,
  EXECUTE) rather than the copy-engine group the service already understood,
  so those releases were never applied. Decode both forms.

- Emulated vidmem: channel blocks (USERD/instance) and some client semaphores
  live in the emulated frame buffer, whose VAs are a window over the BAR1
  backing rather than guest RAM; access them through the right aperture.

- Latency: the service's full-RAM sweep took seconds per pass, so guests'
  progress waits took whole seconds and the driver tore its own setup back
  down in the meantime. Sweep guest RAM in bounded, chunked windows on every
  pass, remember the pages pushes are actually found in and revisit those
  first, and keep the VA->PA map complete (dedup, sort, binary search) so
  semaphores resolve reliably.

Also add a vidmem poke parameter for bringing up and debugging emulated
frame-buffer state.

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