Update PolicyEngine branding

[MaxGhenis]

Summary

• replace remaining Cosilico/CosilicoAI naming with PolicyEngine/microplex branding
• update docs, benchmark artifacts, target-source labels, and package metadata
• repair the legacy engine benchmark so it no longer says PolicyEngine vs PolicyEngine after the migration

Checks

• git grep -i cosilico: clean
• uv run ruff check benchmarks/vs_policyengine.py
• uv run python -m py_compile selected changed Python surfaces
• uv run --with duckdb --with pytest --with responses python -m pytest -q tests/test_supabase_client.py

Note: the repo-wide ruff command still reports pre-existing lint debt outside this branding pass.