[codex] Split PE rebuild CLI parsing from runtime imports

[anth-volk]

Fixes #146

Summary

• Keeps the package-root export surface lazy using the repo's existing __getattr__ pattern so importing microplex_us does not import core microplex or torch-backed surfaces.
• Replaces the checkpoint module _LazySymbol workaround with an explicit parser/runtime boundary: build_policyengine_us_data_rebuild_checkpoint_parser() builds argparse without runtime imports, and execution helpers call _load_runtime_symbols() before doing real work.
• Adds a subprocess regression test that runs the checkpoint module as __main__ with --help, blocks torch, and asserts core microplex was not imported before argparse exits.

Root Cause

The PE-US rebuild CLI was blocked before argument parsing because importing microplex_us and the checkpoint module eagerly imported runtime pipeline modules that reached core microplex. Core microplex.__init__ currently imports torch-backed surfaces, so a no-torch environment could not even render CLI help.

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 tests/test_package_imports.py -q -> 5 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

• I did not rerun the broader checkpoint runtime test module because it exercises full runtime paths that still depend on the core environment. This PR is scoped to import and CLI-help safety.