vgsp-sema: rewind last_get on a fresh channel's second put drop (ISSUE-0002)

r73 c3 rank2: discovery read the previous session's residual put (67); the
pump consumed the live entries 0..1, the r48 resync handled the first drop,
but the guest's counter then settled at put=1 while last_get=2 and
userd_wait_fresh was already consumed. The loop condition last_get != put
then marched ~1023 slots the producer had not written, holding each stale
entry for the full CE grace (one fake release per 8 s) - the client looked
wedged and the exec timed out.

When a drop leaves the pump ahead of the producer on a channel with fewer
than half a ring of submissions, rewind last_get to put instead of marching.
A genuine ring wrap needs a full ring of submissions, so this cannot skip a
live entry.

0 files changed, 0 insertions, 0 deletions