feat(fusion_accounting_reports): line_resolver service for report row computation

Pure-Python helper that resolves a fusion.report's line_specs against
account_totals -> ordered list of report row dicts. Supports three spec
types: account_type_prefix (sum accounts by type), account_id (single
account, drill-downable), and compute='subtotal' (sum last N rows).

Comparison-period support: variance_pct computed automatically when
comparison_totals are supplied.

5 new tests, 32 total passing.

Made-with: Cursor

fusion_accounting_reports/tests/__init__.py

@@ -1,3 +1,4 @@
 from . import test_services_unit
 from . import test_currency_conversion
 from . import test_fusion_report
+from . import test_line_resolver