Support project Python version requirements

[codeforester]

Summary

• Add python.requires_python manifest support with Base's Python 3.10 through 3.13 policy window.
• Report unsupported requirements separately from supported-but-unavailable interpreters through BASE-P170 and BASE-P171.
• Use the selected interpreter when creating Base-managed project virtualenvs, and stop on wrong-version existing venvs until --recreate-venv is used.
• Document the Python runtime contract in docs and AI context.

Validation

• env PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m unittest discover cli/python/base_setup/tests
• env PYTHONPATH=lib/python:cli/python /Users/rameshhp/.base.d/base/.venv/bin/python -m unittest discover cli/python/base_projects/tests
• env PYTHONPATH=lib/python:cli/python /bin/bash -lc 'rg --files -g "*.py" -0 | xargs -0 /Users/rameshhp/.base.d/base/.venv/bin/pylint --rcfile=.pylintrc'
• git diff --check
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash ./bin/base-test

Demo Impact

• None. Setup/check behavior only.

Fixes #855