Extract core source/target/geography surfaces for country packs

[MaxGhenis]

Summary

• add reusable core source manifests, source capabilities, fusion planning, and canonical target provider/spec surfaces
• extend core geography and entity abstractions for country-pack reuse, including benefit-unit support and compatibility shims
• carry the supporting synthesizer/tests needed by the new microplex-us and microplex-uk packages

Verification

• uv run ruff check src/microplex/core/init.py src/microplex/core/entities.py src/microplex/core/source_manifests.py src/microplex/core/sources.py src/microplex/fusion/init.py src/microplex/fusion/planning.py src/microplex/geography.py src/microplex/synthesizer.py src/microplex/targets/init.py src/microplex/targets/provider.py src/microplex/targets/spec.py tests/core/test_sources.py tests/fusion/test_planning.py tests/targets/test_provider.py tests/targets/test_spec.py tests/test_geography_shims.py tests/test_package_surface.py tests/test_synthesizer.py
• uv run pytest -q tests/core/test_sources.py tests/fusion/test_planning.py tests/targets/test_provider.py tests/targets/test_spec.py tests/test_geography_shims.py tests/test_package_surface.py tests/test_synthesizer.py