Add UK firm calibration targets

[MaxGhenis]

Summary

• add ONS UK Business 2025 detailed enterprise-count targets by SIC division and turnover band
• add ONS UK Business 2025 detailed enterprise-count targets by SIC division and employment band
• add HMRC Annual UK VAT Statistics 2024-25 VAT-registered trader count and net VAT liability targets by SIC/trade sector
• expand the uk_firms Ledger target profile with the detailed firm target families and Populace/Axiom bindings
• preserve the existing national turnover/employment target packages used for aggregate checks

Companion PRs

• Populace firm migration: Add experimental UK firm generator populace#223
• Axiom VAT rules: Add GOV.UK VAT rules TheAxiomFoundation/rulespec-uk#65

Testing

• uv run ruff check arch/source_package.py tests/test_arch_source_package.py tests/test_policyengine_ledger_target_profiles.py
• uv run python -m pytest tests/test_arch_source_package.py
• uv run python -m pytest tests/test_policyengine_ledger_target_profiles.py
• cross-repo smoke: emitted 1,439 consumer facts from the four UK firm source packages and loaded them into populace.build.uk_runtime.firm_generation.uk_firm_source_data_from_ledger_facts