refactor: extract _version.py from __init__.py (PR-3/8) (#2550)

* refactor: extract _version.py from __init__.py (PR-3/8)

Move version-checking helpers and `specify self` sub-commands into a
focused `_version.py` module.

Moved symbols:
- GITHUB_API_LATEST — GitHub releases API endpoint constant
- _get_installed_version — importlib.metadata-based version lookup
- _normalize_tag — strip leading 'v' from release tag strings
- _is_newer — PEP 440 version comparison
- _fetch_latest_release_tag — single outbound call to GitHub API
- self_app — Typer sub-app for `specify self`
- self_check, self_upgrade — `specify self check/upgrade` commands

Dependency rule: _version.py imports only stdlib + packaging + ._console.

Backward compatibility: GITHUB_API_LATEST, self_check, self_upgrade
remain importable from specify_cli via re-exports in __init__.py.

Update test_upgrade.py to import helpers from specify_cli._version and
patch at the correct module path (specify_cli._version.*).
Add test_version_imports.py as regression guard.

* fix(tests): update _fetch_latest_release_tag import path in test_authentication.py

PR-3 moved _fetch_latest_release_tag from specify_cli into
specify_cli._version. test_upgrade.py was updated at the time, but
test_authentication.py::TestFetchLatestReleaseTagDelegation still
imported from the old location, causing ImportError on all three
delegation tests. Update all three inline imports to the correct
module path.