This brings fix batch 3c (per-runtime out-buffers, freed on the last
release, plus a steady-state RSS test in its own child process) onto
the branch carrying fix batches 3a and 3b and retention.
The merge was automatic. Checked by hand: release() frees the reused
buffers only after the reference count reaches zero and the pump has
exited. retain() (from 3a) is called only while the provider already
holds a reference, so it cannot resurrect a runtime that is closing.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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