Add declarative US 2024 replacement spec

[MaxGhenis]

Summary

• add packaged microplex_us/specs/us-2024.yaml as the spec-driven MP/eCPS replacement baseline
• declare eCPS clone spine semantics, demographics-only PUF conditioning, full synthetic PUF synthesis, CPS-keep fill, and CPS-keep override steps
• pin microplex to spec-engine commit 90f21d2 and canonical microimpute commit 90be828e
• package the YAML and test it against the existing support-clone constants

Tests

• env -u UV_FROZEN uv run --extra dev python -m pytest tests/specs/test_us_2024_spec.py -q
• env -u UV_FROZEN uv run --extra dev python -m pytest tests/specs/test_us_2024_spec.py tests/test_project_metadata.py -q
• env -u UV_FROZEN uv run --extra dev ruff check tests/specs/test_us_2024_spec.py src/microplex_us/specs/__init__.py
• env -u UV_FROZEN uv build --wheel --out-dir /tmp/microplex_us_spec_wheel_check plus zip check that microplex_us/specs/us-2024.yaml is present

Boundary

This is the declarative spec-pack baseline. It does not promote or rebuild a dataset yet; the next work is wiring run_spec targets/calibrate/export and then running scaled/full candidates against the frozen production eCPS guard.