vgsp: WIP beef ring VAS promote + widen CB scan to draw channel (CONT.399)

- svc_chan_vas_candidate_promote: fall back to svc_channel_owner_vas when
  the GR client channel declares hVASpace==0 (the helper previously returned
  early for exactly the channel whose ring we need to resolve).
- svc_gl_cb_upload_scan: also scan the 0x01xxxxxx RM-channel family (the
  channel that actually replays the draws), not only beef.

NOT YET VERIFIED to resolve the beef ring: the CE siblings register ~19 ms
after the GR channel, and VAS 0xbeef0300 is already 'known' (registered
UNVERIFIED after 1 ms) by then, so the content-verify is skipped. The beef
ring gpfifo=0x4020000 stays ring_pa=0 and the CB scan never runs. See
CONT.399 in handoff-meta-plan.md for the full diagnosis.

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