Anchors a durable, fetchable ref for ATSv2's dependency. The previous pin was
0d8897ae, the tip of codex/pr15-memory-management = refs/pull/51/head: an open,
unmerged, force-pushable PR branch that is typically deleted when the PR merges,
at which point every production install breaks.

Contents beyond upstream main (273d864):
  - the TurboVec scalar-quantization codec, flag-gated and recall-exact
  - MPO chain applied on recall(), not only the pager's cold path
  - recall scope isolation: an empty scoped recall no longer widens to global

557+ tests green at this commit.