Make std_run timeouts enforce runtime bounds

[codeforester]

Context

std_run --timeout N promises bounded execution, but the fallback timeout implementation only sends TERM and then waits for the command. On systems without timeout or gtimeout, a command that ignores TERM can continue running past the requested timeout.

The current macOS development path commonly exercises the fallback because timeout/gtimeout may not be installed.

Reproduction

Force the fallback path by hiding timeout/gtimeout, then run a command that ignores TERM:

PATH="$fake_bin" std_run --no-exit --quiet --timeout 1 /bin/bash -c 'trap "" TERM; sleep 2'

The function returns 124, but only after the process exits naturally. A permanently stuck process would not be bounded.

Scope

• Make std_run --timeout enforce a real runtime bound in the Bash fallback path.
• Align external timeout/gtimeout usage with the same policy where practical.
• Use a clear TERM then KILL escalation strategy with a short grace period.
• Add BATS coverage for a TERM-ignoring command in the fallback path.
• Preserve existing timeout, retry, dry-run, and std_run_with_timeout compatibility behavior.

Acceptance Criteria

• A fallback timeout returns 124 promptly for a command that ignores TERM.
• Timeout-plus-retry still treats 124 as a failed attempt and retries as before.
• Existing simple-command and successful-command behavior is unchanged.
• ./tests/validate.sh passes.