feat: add PDF message exports

[AJV20]

Summary

• Add PDF as a first-class message export format
• Add a native CoreGraphics/CoreText transcript PDF writer with pagination and wrapping
• Wire PDF into iMessage/SMS and WhatsApp export menus/save panels
• Add regression coverage for PDF registration and writer integration

Verification

• swift build
• Scripts/regression/run.py (33 checks)
• Scripts/build.sh
• PDF smoke test generated a valid %PDF-1.3 file with multi-page output
• Scripts/check-pr-scope.sh origin/main

[momenbasel]

Merged into main after fixing the one blocker: exportPDF built its reply-lookup map with Dictionary(uniqueKeysWithValues:), which traps on duplicate GUIDs - and GUIDs come from the untrusted sms.db, so a corrupted or merged backup would crash the app mid-export. Switched to Dictionary(_:uniquingKeysWith:) keeping the first occurrence, and forwarded cancellationCheck into the PDF path so large exports stay cancelable. Two smaller items left as follow-ups: timestamps render every 8th message, and a bubble whose in-line cards exceed a full page can drop trailing body text. Neither blocks the feature.