Remove count aggregation entity metadata

[MaxGhenis]

Summary

• remove the obsolete aggregation.count_entity field from Ledger aggregate facts
• remove the aggregation_count_entity database column and consumer schema property
• regenerate fixture facts/consumer samples so count-valued observations are sums with unit=count
• update stale target-construction docs to describe count targets as sums of count-valued measures

Tests

• uv run ruff check arch/core.py arch/database.py arch/store.py arch/consumer_contract.py tests/test_arch_source_package.py tests/test_arch_database.py tests/test_arch_consumer_contract.py
• uv run pytest tests/test_arch_source_package.py::test_source_package_validation_rejects_count_aggregation tests/test_arch_database.py::test_build_arch_db_writes_aggregate_fact_constraints_and_lineage tests/test_arch_consumer_contract.py::test_checked_in_consumer_fact_sample_matches_exporter tests/test_arch_consumer_contract.py::test_generated_consumer_fact_export_matches_schema tests/test_arch_consumer_contract.py::test_export_consumer_facts_cli_writes_fixture
• uv run pytest tests/test_arch_core.py tests/test_arch_database.py tests/test_arch_consumer_contract.py tests/test_arch_source_package.py