Concurrency cap trimmed to the real fleet size. Each slot is 111.0 MiB of
O(1) state, so this hands ~111 MiB back on a host sitting at 7.1 GiB left
for everything else.
Note the slots already oversubscribed the pool (4 x 262144 > 524288), so
admission was pool-bound regardless: at most two full-length conversations,
or four at 131k. This does not change how much context fits -- it caps
simultaneous requests at three, and a fourth now queues.
Frank
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