refactor: extract _assets.py and _utils.py from __init__.py (PR-2/8) (#2543)

* refactor: extract _assets.py and _utils.py from __init__.py

Move bundle path resolution and version lookup into _assets.py (stdlib only,
zero internal imports), and system utilities (subprocess, tool detection,
file operations) into _utils.py (imports only from ._console). Re-export all
moved symbols from __init__.py for backward compatibility. Update
test_check_tool.py to patch both specify_cli and specify_cli._utils namespaces
since constants are now defined in _utils.

* style: apply PR-1 review patterns to _assets.py and _utils.py

- Add module docstring to _assets.py (stdlib-only, zero internal imports)
- Add blank line after `from __future__ import annotations` in both files
- Replace `Optional[X]` with `X | None` throughout _utils.py (PEP 604)
- Remove unused `Optional` import from _utils.py
- Use explicit re-export form (`X as X`) for public symbols in __init__.py
- Remove unused `subprocess` and `tempfile` imports from __init__.py (moved to _utils.py)