feat: v0.5 — add CSV and PDF export for signal reports and backtest results

[Codex-Crusader]

Summary

The ROADMAP.md v0.5 milestone includes CSV and PDF export of signal reports and backtest results. This is a local-app-only feature (the web demo does not support file I/O).

Deliverables

• CSV export — export the current scan summary (all 24 assets + any custom tickers) as a flat CSV file
• PDF export — export the full analysis view for a selected asset as a formatted PDF (signal score, components breakdown, top news headlines, backtest summary)
• Backtest CSV export — export the full hit-rate table from evaluate_signal_accuracy as CSV

Implementation notes

• Export functions should live in a new pulseengine/local/export.py module (referenced in the ROADMAP v1.0 target structure)
• For PDF generation consider reportlab or fpdf2 (both are pure Python, no system deps)
• For CSV use the stdlib csv module or pandas.DataFrame.to_csv
• The Streamlit download button (st.download_button) can serve the generated bytes directly—no temporary file needed
• Locked-feature prompt in pulseengine/web/dashboard.py should show a “Download the local app to export” CTA

Acceptance criteria

1. Clicking “Export as CSV” from the scan summary view downloads a valid UTF-8 CSV file
2. Clicking “Export as PDF” from the asset analysis view downloads a readable single-page PDF
3. Neither export function writes to disk; all bytes are held in memory and served via st.download_button
4. Export is only available in pulseengine/local/; the web dashboard shows a locked-feature prompt