feat(f11): SLICE-2 — block-level data-src emission (#43, INV-36)

The pure render layer now emits data-src-start/data-src-end (source char offsets
from BlockWithRange) on every LIVE-source rendered block, in BOTH modes — the
contract the webview's selection→source mapping (SLICE-4) walks the DOM for.
Per spec §6.1 INV-36 / §7.2 SLICE-2.

- trackChangesModel: shared `srcAttr(blk)` helper; threaded through renderOp +
  renderReviewOp + the renderReview loop (removed blocks → "" / no data-src, as
  they have no live source). renderPlain switches from a single whole-document
  markdown pass to per-block `<div data-src-start/end>` wrappers — bare divs (no
  cw- class) so the off/clean preview stays visually clean while becoming a
  selection→source surface. Pure, vscode-free, deterministic (extends INV-22).
- unit (test/trackChangesModel.test.ts): data-src offsets equal
  splitBlocksWithRanges in both modes; removed + proposal blocks carry none;
  off-mode stays cw--free; determinism.

200 unit + 47 host E2E green.

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

src/trackChangesModel.ts

@@ -247,7 +247,17 @@ function chip(message: string): string {
   return `<div class="cw-error">Could not render this block: ${md.utils.escapeHtml(message)}</div>`;
 }
 
-function renderOp(op: BlockOp, render: (src: string) => string): string {
+/**
+ * F11 (INV-36): the source-range attributes a live block's wrapping element
+ * carries, so a preview selection can map back to a source markdown range. `""`
+ * when the block has no live source (a baseline-only deletion / a proposal
+ * block) — those are skipped by the selection mapper. Pure + deterministic.
+ */
+function srcAttr(blk: { start: number; end: number } | undefined): string {
+  return blk ? ` data-src-start="${blk.start}" data-src-end="${blk.end}"` : "";
+}
+
+function renderOp(op: BlockOp, render: (src: string) => string, src = ""): string {
   const safe = (src: string): string => {
     try {
       return render(src);
@@ -293,7 +303,7 @@ function renderOp(op: BlockOp, render: (src: string) => string): string {
       }
       break;
   }
-  return `<div class="cw-blk ${cls}">${badge}${inner}</div>`;
+  return `<div class="cw-blk ${cls}"${src}>${badge}${inner}</div>`;
 }
 
 export type AuthorKind = "claude" | "human";
@@ -367,14 +377,25 @@ export function colorByAuthor(
   return sentinelsToSpans(render(injected));
 }
 
-/** Off-state body: the current buffer as plain markdown, no annotations (INV-33). */
+/**
+ * Off-state body: the current buffer as plain markdown, no annotations (INV-33).
+ * Each block is wrapped in a bare `<div data-src-start/end>` so the off-mode
+ * preview is still a selection→source mapping surface (INV-36) while staying
+ * visually clean — no `cw-` annotation classes. A doc with no blocks (empty /
+ * whitespace-only) renders whole. Pure + deterministic.
+ */
 export function renderPlain(currentText: string, opts: RenderOptions = {}): string {
   const render = opts.render ?? defaultRender;
-  try {
-    return render(currentText);
-  } catch (err) {
-    return chip(err instanceof Error ? err.message : String(err));
-  }
+  const safe = (src: string): string => {
+    try {
+      return render(src);
+    } catch (err) {
+      return chip(err instanceof Error ? err.message : String(err));
+    }
+  };
+  const blocks = splitBlocksWithRanges(currentText);
+  if (blocks.length === 0) return safe(currentText);
+  return blocks.map((b) => `<div${srcAttr(b)}>${safe(b.raw)}</div>`).join("\n");
 }
 
 export interface ProposalView {
@@ -411,11 +432,13 @@ function renderReviewOp(
   op: BlockOp,
   render: (src: string) => string,
   colored: (raw: string) => string,
+  src: string,
 ): string {
   // removed blocks and any changed block (atomic fences diffed whole; non-atomic prose
   // word-merged) render via renderOp — no author sentinels (deletions neutral, spec §6.7).
-  if (op.kind === "removed" || op.kind === "changed") return renderOp(op, render);
-  return `<div class="cw-blk ${op.kind === "added" ? "cw-added" : "cw-unchanged"}">${colored(op.block.raw)}</div>`;
+  // `src` is "" for a removed block (no live source — INV-36).
+  if (op.kind === "removed" || op.kind === "changed") return renderOp(op, render, src);
+  return `<div class="cw-blk ${op.kind === "added" ? "cw-added" : "cw-unchanged"}"${src}>${colored(op.block.raw)}</div>`;
 }
 
 /**
@@ -471,7 +494,7 @@ export function renderReview(
     const blk = op.kind === "removed" ? undefined : ranges[ci++];
     const colored = (raw: string): string =>
       blk ? colorByAuthor(raw, blk.start, authorSpans, render) : render(raw);
-    bodyParts.push(renderReviewOp(op, render, colored));
+    bodyParts.push(renderReviewOp(op, render, colored, srcAttr(blk)));
     const here = blockIndex >= 0 ? byBlock.get(blockIndex) : undefined;
     if (here) for (const p of here) bodyParts.push(proposalBlockHtml(p, render));
   }