Answers the open review ask on #109359 with the corrected contract: a hung
remote add_memory is bounded by the SDK timeout (empirically 1.03s at
timeout=1.0, max_retries=0), so shutdown's flush waiting on the capture
lock is bounded, not forever. The guard: while the worker owns the write
(blocked inside add_memory), a concurrent shutdown must wait and must not
re-send the same pending batch. Mutation-checked: lock -> nullcontext
makes this test and the session-switch interleaving test fail.
f7e0ae2aff