Contribute rewrite Phase 4: §8.10 gutter accent in CM6 raw pane

Restores the §8.10 paragraph-margin marker layer Phase 1 dropped when
Tiptap left, this time against the CM6 raw pane on the left half of
the Contribute split. The matching inline tracked-insert/tracked-
delete overlay Phase 3 shipped lives on the preview pane to the
right; the two visual layers answer different questions on the two
halves of the split — "did anything change in this region?" (gutter,
amber, scannable) vs. "what changed here?" (inline, green/red,
precise) — and are deliberately separate signals.

New file: frontend/src/components/diffGutterExtension.js. The
extension exposes a `setBaseline` StateEffect and a gutter that marks
every line whose text differs from the same-indexed line of the
baseline (the last server-confirmed branch body). Per-line, not
paragraph-block — CM6's natural unit; collapsing to paragraph ranges
is more spec-faithful but adds code, and the per-line stance is the
pre-fancy default. A TODO is left for a future paragraph-collapse
pass if the result reads noisy.

MarkdownSourceEditor.jsx changes:
- Install the gutter extension in the editor's extension list.
- Seed the baseline to `initialDoc` at construct time.
- In the existing `initialDoc`-watching effect, dispatch the doc
  replacement AND a `setBaseline` effect in the SAME transaction so
  there's no one-frame window where the new doc reads as "divergent"
  against the old baseline. This carries through every server-
  confirmed branch refresh that RFCView already wires (accept,
  decline, manual flush, branch switch); no RFCView changes needed
  because all four paths already re-push `initialDoc` after pulling
  fresh state.

Design calls per the Phase 4 prompt's open list:
  • Per-line marks, not paragraph-block ranges. Pre-fancy stance.
  • Amber (#f59e0b) thin 3px vertical bar, distinct from the
    preview pane's green-on-light / red-on-light tracked-change
    inline overlay. Reads as "in-flight / not yet on the server."
  • Baseline reset on every branchView refresh (accept / decline /
    manual flush / branch switch), matching RFCView's existing
    originalSourceLinesRef discipline. Gutter then reads as "what's
    in the buffer but not on the server."
  • No hover / no click. The inline overlay already carries the
    click-to-card binding; the gutter is scannable only.

Known caveats kept deliberately:
  • Insert/delete shifts. Adding a line in the middle shifts every
    subsequent line's index and the naive compare lights up
    everything below — tolerated as the honest "you've touched
    stuff below this point" cue. A future LCS-anchored pass could
    fix it; Phase 4 doesn't need to.

SPEC §8.10:
- First paragraph rewritten to name the CM6 raw pane as the gutter's
  home and replace the stale "ProseMirror plugin" wording with the
  CodeMirror gutter framing. Mirrors how Phase 3 named the preview
  pane as the inline overlay's home.
- DiffView retirement paragraph adjusted: gutter and inline overlay
  together (across the split) cover its review affordance, not
  "both layers in the same surface" — the layers are deliberately on
  different surfaces.

Verification:
- Vite preview sandbox eval — standalone CM6 mount, dispatch tests
  across construct (no marks on identity), per-line edit (mark on
  exactly the touched line, confirmed by Y-coordinate matching the
  line-number gutter element), baseline reset clears, atomic
  doc+baseline dispatch leaves zero marks (the RFCView accept-flow
  path), insert-in-middle exhibits the expected cascade, and
  computed-style proof for the amber bar (`#f59e0b`, 3px wide,
  positioned left of the line-numbers gutter at x=21 vs x=24).
- Screenshot captures the bar on the touched line only.
- 125 backend integration tests still green.
- Live RFCView accept → branch refresh → gutter clear flow against a
  real branch was not driven (backend not running this session,
  carrying forward Phase 2/3's verification gap). The sandbox-level
  proof covers the atomic dispatch correctness; the next session
  with a backend up should drive that golden path before piling
  Phase 5 work on top.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SPEC.md

@@ -829,18 +829,20 @@ declined predecessor still visible.
 
 ### 8.10 Tracked-change markup and the review-mode toggle
 
-Two visual layers carry change information in the editor. The first
-is a paragraph-margin marker — a thin gutter accent on any paragraph
-that differs from the branch's open-session baseline, rendered by a
-ProseMirror plugin against a baseline snapshot taken when the editor
-opens. The second is inline `tracked-delete` / `tracked-insert`
-markup at the exact range of an accepted change, with the deleted
-text shown struck-through and the inserted text shown in an additive
+Two visual layers carry change information in the Contribute split.
+The first is a paragraph-margin marker on the left raw-source pane —
+a thin gutter accent on any line that differs from the branch's
+open-session baseline, rendered by a CodeMirror gutter extension
+against a baseline reset on every server-confirmed branch refresh
+(accept, decline, manual flush, branch switch). The second is inline
+`tracked-delete` / `tracked-insert` markup on the right preview pane
+at the exact range of an accepted change, with the deleted text
+shown struck-through and the inserted text shown in an additive
 style; both carry the change's ID via a data attribute, enabling the
 click-to-card binding from §8.8. The margin marker is scannable
 ("did anything change in this region?"); the inline markup is
 precise ("what changed here?"). The two answer different questions
-and both are kept.
+on the two halves of the split and both are kept.
 
 The inline markup lives in the rendered preview surface, not on the
 writable editor. With the Contribute-mode split (§8.3) the raw markdown
@@ -865,10 +867,9 @@ selection-quote that drove the change, and the AI's `reason`.
 DiffView is the legacy read-only render surface invoked via the
 Contribute-mode toolbar toggle (§8.15) — a full-editor swap-in that
 reads the same `changes` table. It is retained as an interim path while
-the preview-pane overlay matures and is slated for retirement once
-gutter markers (the §8.10 paragraph-margin layer) land in the preview
-itself; at that point both visual layers live in the same surface and
-the toolbar toggle collapses.
+the split-pane layers mature, and is slated for retirement once
+the raw-pane gutter and the preview-pane inline overlay together cover
+its review affordance; at that point the toolbar toggle collapses.
 
 ### 8.11 Manual edits and collisions with AI proposals