[codex] Avoid torch for PE rebuild import and help

[anth-volk]

Fixes #146

Summary

• Makes the microplex_us package root resolve convenience exports lazily, including the previously eager microplex.targets and validation/geography-facing exports.
• Keeps the PE-US rebuild checkpoint module import-light by replacing heavy module-level imports with thin lazy wrappers and local runtime imports.
• Splits checkpoint CLI parser construction into build_policyengine_us_data_rebuild_checkpoint_parser() so --help exits before stage/runtime execution imports are needed.

Validation

• uv run --no-sync ruff check src/microplex_us/__init__.py src/microplex_us/pipelines/pe_us_data_rebuild_checkpoint.py tests/test_package_imports.py
• uv run --no-sync pytest -q tests/test_package_imports.py tests/pipelines/test_pe_us_data_rebuild_checkpoint.py -> 28 passed
• uv run --no-sync python -m py_compile src/microplex_us/__init__.py src/microplex_us/pipelines/pe_us_data_rebuild_checkpoint.py tests/test_package_imports.py
• uv run --no-sync python -m microplex_us.pipelines.pe_us_data_rebuild_checkpoint --help

Notes

• This replaces [codex] Split PE rebuild CLI parsing from runtime imports #154 with a narrower implementation. It avoids the broad _load_runtime_symbols() design and keeps existing checkpoint monkeypatch seams working via lazy wrapper functions.
• GitNexus MCP tools were not available in this session, and no local .gitnexus index files were visible in this worktree, so I used targeted rg caller/import checks plus git diff --check for the impact/scope fallback.