docs: describe the sharded bash 3.2 job and timing summary in CONTRIBUTING and the bash 3.2 how-to #666

🤖: AI agent (First Mate), AI-assisted; human review required.

Problem

Two pull requests add developer tooling whose usage notes were left out of the files they would naturally edit, to avoid conflicts between the two PRs and the workflow change that depends on them:

• feat(scripts): add summarize-bats-timing for bats TAP timings #652 adds scripts/summarize-bats-timing.
• feat(test): add local bash 3.2 test runner #653 adds scripts/test-acq-bash32 and docs/howto/bash32-testing.md.

The follow-up edits touch CONTRIBUTING.md and docs/howto/bash32-testing.md. They describe the sharded bash 3.2 job (ACQ_BATS_SHARD, the matrix legs, where the per-leg timing summary appears) alongside the local runner. A prepared diff exists in the worktree that holds the workflow branch, in tmp/ci-bash32-matrix-patches/9999-optional-followup-contributing-and-howto.diff (untracked there).

Trigger

Apply after #652 and #653 are both merged. Check that it still applies to the merged text; the sharded-job wording should match whatever the workflow change finally says.

Acceptance

CONTRIBUTING.md and docs/howto/bash32-testing.md describe the sharded job and the timing summary accurately, link check and markdown lint pass.