Add US Supabase target loader

[MaxGhenis]

Summary

• add the US Supabase calibration target loader under microplex-us
• keep the import path lightweight at microplex_us.supabase_targets so it does not pull in PolicyEngine/HDF5-heavy target adapters
• add focused tests for loading, filtering, CPS-column mapping, state target constraints, summaries, and missing credentials
• include the new test in the existing site-snapshot CI test set

Verification

• UV_FROZEN=1 uv run --extra dev ruff check src/microplex_us/supabase_targets.py tests/targets/test_supabase.py
• UV_FROZEN=1 uv run --extra dev pytest -q tests/targets/test_supabase.py
• UV_FROZEN=1 uv run --extra dev --with pydantic --with-editable ../microplex pytest -q tests/test_package_imports.py tests/targets/test_supabase.py tests/pipelines/test_check_site_snapshot.py tests/pipelines/test_imputation_ablation.py tests/pipelines/test_site_snapshot.py tests/pipelines/test_version_benchmark.py
• npx gitnexus impact SupabaseTargetLoader --direction upstream --include-tests --repo /private/tmp/microplex-us-boundary-supabase-targets
• npx gitnexus detect-changes --scope staged --repo /private/tmp/microplex-us-boundary-supabase-targets