feat(fusion_accounting_reports): drill_down_resolver service

Pure-Python helper that, given an account_id and a date range, fetches
posted account.move.line records and returns a flat list of dicts ready
for the drill-down OWL dialog. Used by the engine's drill_down() method.

3 new tests, 35 total passing.

Made-with: Cursor

fusion_accounting_reports/services/__init__.py

@@ -3,3 +3,4 @@ from . import account_hierarchy
 from . import totaling
 from . import currency_conversion
 from . import line_resolver
+from . import drill_down_resolver