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Swap the Contribute-mode editing surface from Tiptap WYSIWYG to a CodeMirror 6 markdown source editor. Discuss mode and any read-only viewing continues to render through Tiptap. The §8.11 manual-edit debounce now reads the raw markdown source via CodeMirror's doc, eliminating the lossy `editor.getText()` round-trip that RFCView.jsx flagged as a §19.2 candidate; what the contributor typed is exactly what gets POSTed to manual flush. The §8.10 paragraph-margin gutter accent on Tiptap is dropped — it was dead in read-only Discuss anyway, and Phase 4 of the Contribute rewrite adds a change-anchored gutter against the CM6 raw pane. The Tiptap-side accept-time injection of <span class="tracked-*"> markup also drops out — CM6 can't render those spans, and Phase 3's preview pane is the proper home for tracked changes. The reviewMode / DiffView toggle still works against marked-rendered HTML in the interim until Phase 7 retires it. Notes: - Bundle gzipped grows ~50KB for CM6 modules (state/view/commands/ language/lang-markdown). Mermaid in Phase 2 will be the larger lift and should lazy-load. - Verified the CM6 editor mounts cleanly via Vite dev preview: ref handle (`view/getDoc/setDoc`) is wired, `onUpdate` fires on doc changes, gutter / line-numbers / active-line all render, and the source round-trips verbatim. Could not drive the full RFCView Contribute flow end-to-end without a running backend; the manual countdown + save-now + accept/decline pathways are verified by inspection only. - 125 backend integration tests still green. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>