#48: pinning the baseline leaves the review panel fully un-annotated

A pinned baseline with no changes since should read as a clean starting point,
but the F10 on-render still author-colored every block (colorByAuthor), painting
the whole document green/blue right after a pin. Now, when the baseline reason is
"pinned" and there are zero changes since (every diffBlocks op unchanged), the
on-render is fully clean — no change marks and no authorship coloring — while the
data-src block mapping (INV-36) and any pending proposals (review actions, not
annotations) are kept.

Scoped to the PIN specifically, not all zero-diff: a baseline advanced by a
machine-landing (accept) is also zero-diff but keeps its authorship coloring so
accepted Claude text stays blue (F10 INV-33). renderReview takes a `pinned`
option; the controller passes baseline.reason === "pinned" from both refresh and
the renderHtmlFor test seam.

- trackChangesModel.ts: renderReview gains the `pinned` RenderOption; when pinned
  + zero-diff, blocks render plain (no colorByAuthor).
- trackChangesPreview.ts: pass { pinned } through refresh + renderHtmlFor.
- unit: pinned+zero-diff → no cw-by-*; pinned+zero-diff still shows proposals;
  zero-diff WITHOUT pin (machine-landing) keeps coloring (INV-33); real changes
  after a pin re-color.
- s48PinClean host E2E: type → colored; pin → clean; edit → annotations return.

218 unit + 74/5 host E2E green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/trackChangesPreview.ts

@@ -337,7 +337,7 @@ export class TrackChangesPreviewController implements vscode.Disposable {
     void panel.webview.postMessage({
       type: "render",
       mode,
-      html: renderReview(baselineText, current, spans, proposals),
+      html: renderReview(baselineText, current, spans, proposals, { pinned: baseline?.reason === "pinned" }),
       epoch: this.epochLabel(baseline),
       summary,
       authorable,
@@ -464,6 +464,7 @@ export class TrackChangesPreviewController implements vscode.Disposable {
       current,
       this.attribution.spansFor(doc),
       this.proposals.listProposals(doc),
+      { pinned: baseline?.reason === "pinned" },
     );
   }
   /** F10: current annotations mode for a panel (default on). */