ci(freebsd): add build-freebsd job to reusable build workflow

[PR9000]

What does this PR do?

Extracted from PR #1138 as requested during review.

Adds FreeBSD 15.1 and 14.4 x86_64 build matrix legs to the reusable build workflow (.github/workflows/_build.yml).

Key details on the vmactions FreeBSD setup:

• Runner Host: Pinned to ubuntu-latest (x86_64 host). Running QEMU guest emulation on ARM runners (ubuntu-*-arm) causes severe performance degradation leading to 3+ hour timeouts.
• VM Action & Sync: Uses vmactions/freebsd-vm@v1 with sync: rsync.
• Environment Setup: Packages required for build and frontend (gmake, sqlite3, pkgconf, node22, npm-node22, bash) are installed natively inside the FreeBSD VM via pkg.
• Disabled aarch64 leg: The aarch64 leg remains commented out because lightningcss (a native Rust dependency of the Vite/CSS pipeline) does not publish prebuilt binaries for freebsd-arm64, causing npm run build to fail.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.