Three follow-ups from review on #28296:
1. Sync worker outliving the bounded join. Each sync_turn POST has
_TIMEOUT=30s and there are two per turn, but on_session_end and
on_session_switch only join for 10s. If the worker is still alive
after the join, committing the old session orphans the worker's
late writes past the commit boundary — they land in an already-
committed session and never get extracted. Both hooks now re-check
is_alive() after the join and skip the commit when the worker
hasn't drained.
2. on_memory_write late session_id capture. Same shape as the
pre-fix sync_turn: f-string for the post path read self._session_id
inside the worker, so a switch between thread spawn and post call
landed the memory note in the new session. Snapshot sid at call
time, same pattern as sync_turn.
3. Stale prefetch repopulating the new session. The pre-switch
drain+clear only protects against workers that finish before the
join completes; one finishing after the clear would write its
result into the new generation's slot. Added a monotonic
_prefetch_generation; workers capture it at spawn and refuse to
write if it has advanced.
Tests: existing in-flight-sync test updated to drain (it tested the
join-before-commit happy path); four new tests cover hung-writer skip
on end + switch, on_memory_write sid capture, and prefetch generation
gating. 177/177 memory tests pass.
(cherry picked from commit 3791a87dbea518b06fc9e2e8e2da69e21a11cb41)
eddbf291a4