Add UK firm calibration targets

[MaxGhenis]

Summary

• add ONS UK Business 2025 firm enterprise count targets by turnover and employment band
• add HMRC Annual UK VAT Statistics 2024-25 VAT-registered trader count and net VAT liability targets by turnover band
• add a uk_firms Ledger target profile with Populace and Axiom bindings
• preserve underscore-bearing delimited identifiers as strings so employment bands like 10_19 do not parse as numbers

Companion PRs

• Axiom VAT rules: Add GOV.UK VAT rules TheAxiomFoundation/rulespec-uk#65
• Populace firm support plan: Document UK firm support plan populace#199

Testing

• uv run pytest tests/test_arch_source_cells.py::test_delimited_text_parsers_preserve_underscore_identifiers tests/test_arch_source_package.py::test_ons_uk_business_firm_targets_source_package_alias_validates_counts tests/test_arch_source_package.py::test_ons_uk_business_firm_targets_source_package_builds_firm_facts tests/test_arch_source_package.py::test_hmrc_vat_firm_targets_source_package_alias_validates_counts tests/test_arch_source_package.py::test_hmrc_vat_firm_targets_source_package_builds_vat_facts tests/test_policyengine_ledger_target_profiles.py::test__given_uk_firms_profile__then_it_declares_ledger_only_firm_targets