Add portable temporary file and directory helpers to lib_std.sh

[codeforester]

Context

Portable mktemp usage is easy to get wrong across macOS and Linux, especially around mktemp -t. lib_std.sh should provide simple temp file and directory helpers that follow Base conventions and integrate with cleanup registration.

Scope

Add Base-native temp helpers to lib_std.sh, built on the cleanup/trap registration API. Do not copy external implementations.

Acceptance Criteria

• Public helpers create a temp file and a temp directory using caller-provided result variables.
• Helpers avoid platform-specific mktemp -t assumptions and work with ${TMPDIR:-/tmp}.
• Temp paths are registered for cleanup by default.
• A clear opt-out exists for callers that intentionally need to keep the temp path.
• Invalid result variable names fail clearly.
• Public API and behavior are documented in lib/bash/std/README.md.
• BATS coverage verifies file creation, directory creation, prefixes, cleanup registration, opt-out behavior, and invalid arguments.

Validation

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