Add stdlib timeout runner

[codeforester]

Summary

• Add std_run_with_timeout with std_run-style --no-exit, --quiet, and dry-run handling.
• Prefer timeout/gtimeout when available and provide a Bash fallback for macOS/Linux portability.
• Document timeout status behavior and add focused BATS coverage.

Validation

• bats lib/bash/std/tests/lib_std.bats
• ./tests/validate.sh
• git diff --check

Stacked on #58.
Fixes #51