Add Ledger API compatibility path

[MaxGhenis]

Summary

• introduce policyengine_ledger as the additive public API alias for the existing arch implementation
• add a ledger CLI entry point that delegates to the current Arch CLI
• update README naming/boundaries toward Ledger, Populace, and Thesis while preserving the historical arch namespace

Validation

• uv run ruff check policyengine_ledger tests/test_policyengine_ledger_imports.py
• uv run python -m pytest tests/test_arch_namespace.py tests/test_arch_facts.py tests/test_arch_consumer_contract.py tests/test_policyengine_ledger_imports.py -q (37 passed)
• uv run arch validate-facts --fixture and uv run ledger validate-facts --fixture produce identical valid 80-fact reports
• uv build --wheel and wheel inspection confirmed all policyengine_ledger files are packaged

Notes

This is intentionally additive. The legacy arch namespace remains the implementation namespace, and existing imports/CLI behavior are preserved.