Make Ledger aggregate facts sum-only

[MaxGhenis]

Summary

• remove count as an allowed aggregate fact aggregation method
• migrate count-valued source-package measures to aggregation: sum with unit: count
• reject aggregation: count during source-package validation
• regenerate Arch fixture facts/consumer sample under the sum-only contract
• keep human labels for sum-over-count facts readable as count facts

Validation

• uv run ruff format --check arch/core.py arch/jurisdictions/us/soi.py arch/source_package.py tests/test_arch_database.py tests/test_arch_suite.py tests/test_policyengine_ledger_imports.py tests/test_arch_source_package.py
• uv run ruff check arch/core.py arch/jurisdictions/us/soi.py arch/source_package.py tests/test_arch_database.py tests/test_arch_suite.py tests/test_policyengine_ledger_imports.py tests/test_arch_source_package.py
• git diff --check
• uv run python -m pytest tests/test_arch_core.py tests/test_arch_consumer_contract.py tests/test_arch_database.py tests/test_arch_suite.py tests/test_policyengine_ledger_imports.py -q
• uv run python -m pytest tests/test_arch_source_package.py -q