Adds a TransientModel wizard fusion.xlsx.export.wizard that lets users
pick a report type, date range, and comparison mode, then runs the
engine and produces an XLSX via xlsxwriter (in-memory).
The wizard exposes a download field that becomes available after export
finishes. Works on P&L, Balance Sheet, Trial Balance, and General Ledger.
Comparison columns are written when the engine returns a comparison_period
in the result.
Also wires the controller's /fusion/reports/export_xlsx endpoint to drive
the wizard and return base64-encoded XLSX bytes (replaces the not_implemented
placeholder).
Tests: 2 new (test_xlsx_export.py) + 1 controller test updated. Manifest
declares xlsxwriter as an external_dependency.
Made-with: Cursor
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