Allow checkpoint-only version artifact directories

[anth-volk]

Summary

Fixes #179.

• Allow an explicit version artifact directory to be reused when its existing contents are limited to the configured PE-US rebuild checkpoint trees for the same run.
• Preserve the existing collision behavior for unrelated contents, unconfigured checkpoint contents, missing checkpoint configuration, and non-directory version paths.
• Forward stage_runtime_writer through save_versioned_us_microplex_artifacts() to match the existing _finalize_versioned_build_artifacts() call path.

Notes on Scope

The checkpoint-only directory allowance is the direct #179 fix. While validating the full versioned-artifacts test file, current main also exposed an adjacent finalization mismatch: _finalize_versioned_build_artifacts() already passed stage_runtime_writer, but save_versioned_us_microplex_artifacts() did not accept or forward it. This PR includes that small pass-through so the live stage-runtime path remains consistent.

Tests

• ruff check src/microplex_us/pipelines/versioned_artifacts.py tests/pipelines/test_versioned_artifacts.py
• git diff --check
• PYTHONPATH=/Users/administrator/Documents/PolicyEngine/microplex-us/src PYTHONDONTWRITEBYTECODE=1 /Users/administrator/Documents/PolicyEngine/worktrees/microplex-us/fix-pe-rebuild-smoke-issues/.venv/bin/python -m pytest -q -p no:cacheprovider tests/pipelines/test_versioned_artifacts.py -k 'configured_checkpoint or unrelated_existing or unconfigured_checkpoint or existing_version_file'
• PYTHONPATH=/Users/administrator/Documents/PolicyEngine/microplex-us/src PYTHONDONTWRITEBYTECODE=1 /Users/administrator/Documents/PolicyEngine/worktrees/microplex-us/fix-pe-rebuild-smoke-issues/.venv/bin/python -m pytest -q -p no:cacheprovider tests/pipelines/test_artifacts.py::test_source_provider_versioned_build_initializes_live_stage_writer

A full tests/pipelines/test_versioned_artifacts.py run passed before the final non-directory guard was added; after the guard, the focused allocator tests above cover the changed behavior.