v0.31.1 — Sprint 33 static-path perf fixes (PyPI catch-up)

Sprint 33 static-path perf fixes reach PyPI. v0.31.0 shipped
on 2026-06-04 and predates Sprint 33's static-middleware work; the
three landed PRs below were on master but never made it into a
published wheel. Sprint 34's release-management audit surfaced
the gap (see bench/sprint-logs/sprint-34.md). No new code in
this release — just the cut from the correct git revision.

Changed

• Static cache documents stat()-based invalidation, not
  permanent staleness (PR #47, docs(known-limitations): clarify static cache is stat-invalidated).
• Static cache hits avoid mimetypes.guess_type() regex on every
  request — mime is computed once at cache-store and held in the
  cache entry; stat() is throttled by BB_STATIC_STAT_TTL_S
  (default 1 s); response body + headers go out as a single
  writelines() vectored write instead of two write() calls
  (PR #48, perf(static): cache mime + throttle stat + vectored write on cache hit).
• Static middleware hot path uses os.path instead of
  pathlib.Path — _root is a str; traversal check is a
  single string-prefix comparison against the pre-computed
  <root>/ form; precompressed-sibling existence is
  os.path.isfile(target + suffix); _root: Path is kept as a
  back-compat property (commit 7b63fbe,
  perf(static): replace pathlib with os.path on the hot path).

Notes

• Public surface unchanged. No deprecations; no migration needed.
• v0.32.0 (Sprint 33 close release) will fold in the
  phase-trace API and Compression pass-through fast-path PRs
  currently in review.

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