Contribute rewrite Phase 1: CM6 markdown source editor

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>

frontend/src/components/Editor.jsx

@@ -1,22 +1,18 @@
-// Editor.jsx — the §8 center-column editor.
+// Editor.jsx — the Discuss-mode read-only renderer (and main/no-contribute
+// fallback). Tiptap on ProseMirror per §18; the Contribute-mode editing
+// surface moved to MarkdownSourceEditor (CodeMirror 6) in the Phase 1
+// rewrite of Contribute, leaving this surface for read-only rendering
+// plus the §8.12 selection tooltip's coords.
 //
-// Tiptap on ProseMirror per §18. Two ProseMirror plugins live alongside
-// StarterKit:
-//
-//   • paragraphDiff — the §8.10 paragraph-margin gutter accent. Compares
-//     each paragraph against an open-session baseline (the
-//     `originalParagraphsRef` ref the parent owns and refreshes when the
-//     baseline shifts — e.g. on branch switch or a server-side flush).
+// One ProseMirror plugin still lives alongside StarterKit:
 //
 //   • selectionHighlight — keeps a selected passage highlighted while
 //     focus moves to the §8.12 selection tooltip. Driven by meta
 //     transactions dispatched from the parent.
 //
-// The inline tracked-delete / tracked-insert markup from §8.10 is
-// session-local HTML the parent injects via `editor.commands.setContent`
-// when a change is accepted; the editor itself doesn't own that state.
-// On reload the markup clears and DiffView (toolbar toggle) is the
-// durable read of accepted changes.
+// The paragraph-margin gutter accent that used to live here is dropped;
+// Phase 4 of the Contribute rewrite adds a proper change-anchored gutter
+// against the CodeMirror raw pane.
 
 import { useEditor, EditorContent, Extension } from '@tiptap/react'
 import StarterKit from '@tiptap/starter-kit'
@@ -25,47 +21,6 @@ import { marked } from 'marked'
 import { Plugin, PluginKey } from 'prosemirror-state'
 import { Decoration, DecorationSet } from 'prosemirror-view'
 
-// ── Paragraph diff plugin ────────────────────────────────────────────────
-
-const diffKey = new PluginKey('paragraphDiff')
-
-function makeDiffPlugin(originalParagraphsRef) {
-  return new Plugin({
-    key: diffKey,
-    props: {
-      decorations(state) {
-        const originals = originalParagraphsRef?.current
-        if (!originals || originals.length === 0) return DecorationSet.empty
-
-        const decorations = []
-        let idx = 0
-        state.doc.descendants((node, pos) => {
-          if (node.type.name === 'paragraph' || node.type.name === 'heading') {
-            const current = node.textContent.trim()
-            const original = (originals[idx] ?? '').trim()
-            if (current !== original) {
-              decorations.push(
-                Decoration.node(pos, pos + node.nodeSize, { class: 'paragraph-changed' })
-              )
-            }
-            idx++
-          }
-        })
-        return DecorationSet.create(state.doc, decorations)
-      },
-    },
-  })
-}
-
-function DiffExtension(originalParagraphsRef) {
-  return Extension.create({
-    name: 'paragraphDiff',
-    addProseMirrorPlugins() {
-      return [makeDiffPlugin(originalParagraphsRef)]
-    },
-  })
-}
-
 // ── Selection highlight plugin ────────────────────────────────────────────
 
 export const selectionHighlightKey = new PluginKey('selectionHighlight')
@@ -110,7 +65,6 @@ function SelectionHighlightExtension() {
 export default function Editor({
   content,
   editorRef,
-  originalParagraphsRef,
   onSelectionChange,
   onUpdate,
   editable = true,
@@ -132,7 +86,6 @@ export default function Editor({
   const editor = useEditor({
     extensions: [
       StarterKit,
-      DiffExtension(originalParagraphsRef),
       SelectionHighlightExtension(),
     ],
     content: '<p></p>',
@@ -167,20 +120,10 @@ export default function Editor({
     }
   }, [editor, onSelectionChange, reportSelection])
 
-  // Reload content + snapshot baseline paragraphs.
   useEffect(() => {
     if (!editor || content == null) return
     const html = marked.parse(content)
     editor.commands.setContent(html, false)
-    if (originalParagraphsRef) {
-      const paragraphs = []
-      editor.state.doc.descendants(node => {
-        if (node.type.name === 'paragraph' || node.type.name === 'heading') {
-          paragraphs.push(node.textContent.trim())
-        }
-      })
-      originalParagraphsRef.current = paragraphs
-    }
   }, [content, editor])
 
   return (