Add timeout-aware command execution helper to lib_std.sh

[codeforester]

Context

Scripts sometimes call external tools that can hang. GNU systems often provide timeout; macOS may provide gtimeout through Homebrew coreutils, or no timeout binary at all. lib_std.sh should offer a small wrapper with clear behavior and Base-style command execution semantics.

Scope

Add a Base-native timeout-aware command helper to lib_std.sh. Do not copy external implementations.

Acceptance Criteria

• Public helper runs a command with a timeout while preserving argument boundaries.
• Prefer timeout, then gtimeout, when present.
• Define and test clear behavior when neither timeout binary is available.
• Preserve command exit status on normal command failure.
• Support caller-handled failures in a style consistent with std_run --no-exit.
• Do not use eval.
• Public API and behavior are documented in lib/bash/std/README.md.
• BATS coverage verifies success, command failure, timeout, missing command, missing timeout binary fallback, and invalid arguments.

Validation

• Focused stdlib BATS coverage.
• ./tests/validate.sh
• `git diff --check