Make UK firm Ledger selectors profile-driven

[MaxGhenis]

Summary

• replace hardcoded UK firm Ledger record-set constants with a target-profile selector object
• allow the Ledger-style uk_firms target profile mapping to drive fact record-set selection
• add regression coverage for custom profile record sets and missing profile targets

This keeps the experimental firm generator compatible with today's facts while moving the source wiring toward the eventual spec-driven path. The remaining support-layout assumptions stay in Python for now and are marked as the next piece to lift into declarative specs.

Tests

• uv run --project packages/populace-build --extra uk python -m pytest packages/populace-build/tests/test_uk_firm_generation.py
• uv run ruff check packages/populace-build/src/populace/build/uk_runtime/firm_generation.py packages/populace-build/tests/test_uk_firm_generation.py
• uv run --project packages/populace-build --extra uk python -m pytest packages/populace-build/tests