docs: add final PDF export re-engineering investigation and approved architecture

[ThisIs-Developer]

Motivation

• Deliver a complete, source-backed investigation and approved architecture for re-engineering the PDF export subsystem to address severe performance and layout issues with large documents.
• Provide an evidence-driven, phased replacement plan and acceptance gates so the team can safely replace the current html2canvas/jsPDF raster pipeline without changing editor/preview behavior.

Description

• Add a 677-line final investigation and architecture report at docs/pdf-export-engine-reengineering-report.md containing the required 13 sections and 10 independent specialist agent findings, coordinated and approved by the centralized manager (Agent 0).
• The report documents the current pipeline (duplicate Markdown parsing, off-screen export DOM, Mermaid/MathJax replay, monolithic html2canvas capture, per-page PNG slicing, and jsPDF assembly), root causes, industry comparisons, the recommended architecture, phased implementation plan, and verification matrices.
• The change is documentation-only and does not modify product behavior, preview rendering, or application source files; the new report is intentionally the only artifact added to docs/.

Testing

• Ran git diff --check and project static checks; no formatting or whitespace errors were reported.
• Verified the report structure with rg -c '^## Section (1|2|3|4|5|6|7|8|9|10|11|12|13) —' (returned 13) and rg -c '^### Agent ([1-9]|10) —' (returned 10).
• Performed syntax checks with node --check script.js, node --check preview-worker.js, and node --check desktop-app/prepare.js, all of which succeeded in this environment.
• The mandatory runtime 50/100/250/500/1000-page performance benchmarks were not executed because this checkout lacks a browser executable, prepared desktop runtime libraries, or a GUI benchmark harness; those runs are recorded in the report as required acceptance gates prior to production cutover.

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