feat: add explicit async command adapter

[codeforester]

Summary

• add App.async_command() for explicit async callbacks
• add base_cli.run_async() with adapter-owned loop and nested-loop guard
• preserve the synchronous lifecycle, context, logging, cleanup, and exit-code boundary
• add public API, contract, and lifecycle tests plus documentation

Validation

• python -m pytest (all tests passed)
• ruff check --fix and ruff format --check
• python -m mypy --strict lib/python/base_cli
• scripts/validate_docs.py
• ./tests/validate.sh
• git diff --check

Closes #191