fix(foundation): resume spawn backoff after EINTR (#1616)

[rarepops]

What does this PR do?

Adds cbm_nanosleep_full() to resume POSIX sleeps after EINTR, and uses it for subprocess spawn backoff so transient signals cannot silently shorten the documented retry budget. Other sleep call sites retain their existing interruptible behavior.

Adds a POSIX regression test that repeatedly interrupts the deterministic spawn backoff with SIGALRM and verifies the full cumulative delay is preserved.

Fixes #1616

Checklist

• Every commit is signed off (git commit -s) - required, CI rejects unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (subprocess: 32 passed)
• Lint passes (clang-format-20, cppcheck 2.20.0)
• 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.