vgsp: fail4 r259-r262 - capture ring (4 slots) + diagnostic budgets; CE auto-advance tried and revertedHEADmaster

1. svc_gr_cand is now a 4-slot ring addressed by an accessor macro
   (svc_gr_cand -> ring[pos]); captures that would overwrite a
   DIFFERENT launch's unconsumed candidate move to a free slot, and
   svc_gr_request selects this channel's oldest unconsumed candidate
   before consuming; the defer-arm gate checks any unconsumed
   candidate. Struct tag renamed svc_gr_cand_rec to keep the macro
   safe. (The streams2 cross survives this - it is not (only) the
   single slot; see the new evidence in the handoff.)

2. Diagnostic budgets raised for the triage (opadd/op_trace 2048,
   memsets rc-log 2048). These exposed the real memset shape: the
   8 MB cudaMemset is SIXTEEN grid=1024/block=512 kernel launches
   whose win0 param bank is fully correct (dst patched to the reloc,
   w0x168=0x5a5a5a5a, launches OK) - yet the D2H readback returns
   zeros, so the loss is between the engine's stores and the
   readback pages (the rec hpa/landing class, GRVAUDIT unlanded
   mass), not the params.

3. A CE-style per-op dst auto-advance was implemented, evaluated
   (no memset improvement) and reverted - re-sets are decoded and
   the risk to the green clients was not justified.

Verified on r262 (boot 1 of the acceptance round): ctaid_probe RAWA
0 1 2 3 4 5 6 7 + PROBE2 bad=0; transformer STATUS OK; streams2
bad=65536 (red); memcpy_roundtrip wedged the guest (flaky: green on
r254, wedges on r260/r262).

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