Move Supabase target loader behind core shim

[MaxGhenis]

Summary

• replace the core Supabase target loader implementation with a compatibility shim to microplex-us
• keep an actionable ImportError when microplex-us is not installed
• shrink core tests to public shim behavior

Verification

• UV_FROZEN=1 uv run --extra dev ruff check src/microplex/supabase_targets.py tests/test_supabase_targets.py tests/test_package_surface.py
• UV_FROZEN=1 uv run --extra dev pytest -q tests/test_supabase_targets.py tests/test_package_surface.py
• UV_FROZEN=1 uv run --extra dev ruff check src/microplex/core src/microplex/targets src/microplex/supabase_targets.py tests/targets tests/test_package_surface.py tests/test_supabase_targets.py
• UV_FROZEN=1 uv run --extra dev --with duckdb pytest -q tests/test_package_surface.py tests/test_supabase_targets.py tests/targets/test_benchmarking.py tests/targets/test_reweighting.py
• gitnexus impact: low risk, 0 affected processes
• gitnexus detect-changes: low risk, 0 affected processes

Note: full-repo ruff currently fails on pre-existing unrelated style issues across calibration/model/test files.