Remove aggregation method from Ledger target selectors

[MaxGhenis]

Summary

• remove aggregation_method as a selectable Ledger target selector field
• keep Ledger aggregation only as source lineage metadata and sum-only validation
• update tests so count aggregation remains a rejected input, not a target option

Tests

• uv run --project packages/populace-build --group dev python -m pytest packages/populace-build/tests/test_ledger_targets.py packages/populace-calibrate/tests/test_target.py -q
• uv run ruff check packages/populace-build/src/populace/build/ledger_targets.py packages/populace-build/tests/test_ledger_targets.py packages/populace-build/tests/test_us_fiscal_targets.py
• git diff --check