vgsp-sema: GR fresh-put-drop rewind - stop consuming the previous session's residual ring

r203 battery forensics (rank 0, sema_dbg=1): hang and progress alternate
perfectly with the first USERD GP_PUT read of a fresh session. A session
whose first read is residual (106/107/108 = the previous session's final
put, GET==PUT) consumes the stale ring in its first pump pass, then never
decodes the new client's live entries: its ctx-init completion word (the
channel's descending counter slot) freezes at the first stale-decoded
payload (1), publishes stop (its USERD put reads never advance past 1),
and the client polls forever - multikernel (gen3), streams2 (gen5),
victory_mail (gen7) and smiley (gen9) hung; memcpy (gen4), streams_events
(gen6), victory_matmul (gen8) - each following a session whose final put
was 1 - proceeded. The GR ring/USERD live in fixed VRAM slots
(fb 0x140000 + rank*0x3000) that keep the dead session's state across
svc_session_flush; m176 reset learned counter state but not the ring
bookkeeping.

m178/m179's r48 resync (last_get=0 on the first put drop) applies only
when userd_wait_fresh is set, and the r73 rewind is CE-only
(engine_type != 0), so a GR channel whose fresh put drops below put_prev
keeps last_get ahead of the producer forever. Extend the r73 rewind shape
to GR: rewind last_get to slot 0 (the fresh session restarts its push
sequence at slot 0, and re-decoding slot 0 is idempotent - the
forward-only apply drops the duplicate payload), gated by the same
submissions < gpfifo_entries/2 freshness guard so a genuine wrap is
unaffected.

Live evidence (smiley, killed at 127105): counter page word 0x20400fff0
stuck at 1; ring put=107 get=107 at discovery; pub-adv 107->1; repeated
rel-apply pay=1 -> skip once per pass. Worktree build: make kernel green
(vgsp.ko linked).

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