Wire calibration harness to target providers

[MaxGhenis]

Summary

• let CalibrationHarness consume any core TargetProvider, while preserving TargetRegistry as the default
• add provider-backed target selection with US category/level/group/availability filters
• add a Supabase-provider-to-harness smoke test and include harness tests in focused CI

Verification

• UV_FROZEN=1 uv run --extra dev ruff check src/microplex_us/calibration_harness.py tests/test_calibration_harness.py tests/targets/test_supabase.py
• UV_FROZEN=1 uv run --extra dev pytest -q tests/test_calibration_harness.py 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/test_calibration_harness.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
• GitNexus impact: low risk for CalibrationHarness
• GitNexus detect-changes: medium risk, expected affected flows through run_pe_parity_suite target selection/calibration paths