feat: export comparison reports as CSV and Excel

[yigitcan-ozturk]

What changed

• add --report CLI export for .csv and .xlsx
• keep JSON --output as the machine-readable integration payload
• export ranked supplier comparison data with recommended flags
• add Excel Comparison and Summary worksheets
• include decision summary and effective scoring weights in Excel reports
• expose write_report through the public Python API
• add report export and installed-package API tests
• update documentation and remove report export from the roadmap

Compatibility

• existing CLI output remains unchanged unless --report is supplied
• scoring, weighting and v0.2 JSON integration behavior are unchanged
• report exports are generated from the same scored payload used by JSON output

Why

This turns rfqdiff results into reviewable procurement artifacts without creating a second decision path or duplicating scoring logic.