enhancement: add platform-aware cache paths

[codeforester]

Summary

• add a reusable platform-aware default cache-root resolver;
• honor XDG_CACHE_HOME, macOS Library/Caches, and Windows LOCALAPPDATA;
• normalize home-relative history paths across POSIX and Windows separators;
• isolate Windows-related home/cache environment variables in test helpers;
• document Linux, WSL2, macOS, and Windows cache placement.

This keeps BASE_CLI_CACHE_DIR as the explicit override and leaves consumer-specific
configuration and native Base/basectl support out of scope.

Fixes #36

Validation

• uv run --no-project --with pytest --with click --with PyYAML python -m pytest
• ./tests/validate.sh

105 tests passed.