Add Arch target core helpers

[MaxGhenis]

Summary

• add neutral Arch consumer-fact JSONL accessors for country packages
• add generic tabular rollup target provider utilities
• add vectorized preserved tax-unit table construction for source records with existing unit IDs
• bump package version to 0.2.0 for the new public helper surface

Tests

• uv run ruff check src/microplex/targets/arch.py src/microplex/targets/rollups.py src/microplex/tax_units.py src/microplex/targets/init.py src/microplex/init.py tests/targets/test_arch.py tests/targets/test_rollups.py tests/test_tax_units.py
• uv run --extra dev python -m pytest tests/targets/test_arch.py tests/targets/test_rollups.py tests/test_tax_units.py -q
• uv build