v2026.10.3.1

This release is identical to 2026.10.2.1 in code; the bump exists to publish a
fresh GitHub Release and a fresh index entry, because the 2026.10.2.1 release
ran while the canary runners on ubuntu-24.04 were unavailable and the publish
itself could not complete. It carries no behavioural change and is wire-
and-cache-compatible with 2026.10.2.1: a record written by 2026.10.2.1 is
still admitted by 2026.10.3.1, and a record written by 2026.10.3.1 is admitted
by 2026.10.2.1 (the engine field exists in both; its value 2026.10.x matches
its own self).

CI

• Bootstrap fast path. bootstrap-mcpp skips the xlings tarball fetch,
  extract and self install when the restored cache already holds the pinned
  xlings version. The check is the one case the guard would otherwise miss:
  a stale xlings cache from before the pin was bumped, or a binary that no
  longer runs because its dynamic loader is gone. On Linux it shaved the
  Run ./.github/actions/bootstrap-mcpp step from ~5 s to ~2 s on the
  bootstrap-mcpp invocation per job, summed over ~30 jobs per run; on Windows
  from ~68 s to ~30–60 s. The fast path addresses xlings by $XL_BIN_PATH
  rather than by the bare name, because Git Bash re-derives PATH from the
  Windows environment on every child shell and drops the mixed-separator
  entry the export PATH writes; xlings self install would write it back
  via [Environment]::SetEnvironmentVariable, but the fast path skips that
  step.
• Windows e2e MinGW prewarm guard. Each ci-windows-e2e shard now
  probes the two locations run_all.sh looks at for g++.exe before
  invoking mcpp toolchain install mingw 16.1.0, so a sandbox restored
  from the build job's prewarm does not pay the install call again.
  Measured 68 s → 45 s on shard 1/3; the other two shards were already
  cheaper. The install is still run when the probe finds no payload, so
  a missing-prewarm shard is unchanged.

Operator-facing note (transient)

• release.yml carries a skip_canaries workflow_dispatch input,
  documented in the workflow, kept on the main branch during the
  ubuntu-24.04 runner outage of 2026-10-02. The canaries design (WS10) is
  intact: the input is if: ${{ !inputs.skip_canaries }} on the
  canaries job, and the gate is otherwise unchanged. This release's
  GitHub Release was created via the input. The input is removed by the
  commit that follows the next successful canary run.