feat(f11): SLICE-4 — single adaptive Ask-Claude button + selection mapping (#43, INV-37)

The one Ask-Claude toolbar button now adapts: its label flips on selectionchange
(Edit Selection when live text is selected in the preview, Edit Document
otherwise), and a click resolves the preview selection to a SOURCE range via the
nearest data-src ancestors (INV-36) — the webview's sole mapping duty. A
selection touching no live-source block falls back to document scope. Per spec
§6.5 PUC-2/3 / §7.2 SLICE-4.

- webview (media/preview.ts): nearestSrc() DOM walk + selectionSrcRange()
  block-union; updateAskLabel() on selectionchange + after each render; the
  adaptive click posts { askClaude, scope:"selection", start, end } or falls back
  to document scope. Sealed (INV-21): reads data-src only, posts intent.
- host: runEditAndPropose's range branch (already shared from SLICE-3) records one
  single-range F4 proposal over the resolved block-union.
- host E2E: stubbed selection turn → exactly one proposal over the resolved range
  (turn receives exactly the selected source; replaced == the range; doc
  untouched); an unchanged replacement proposes nothing. (The webview DOM
  selection→data-src lookup is sealed-sandbox → manual smoke, spec §6.8.)

205 unit + 51 host E2E green.

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

media/preview.ts

@@ -39,11 +39,51 @@ pinEl?.addEventListener("click", () => {
   vscodeApi.postMessage({ type: "pinBaseline" });
 });
 
-// F11 (SLICE-3): Ask Claude to Edit Document — post the document-scope intent; the
-// host prompts for the instruction + runs the turn (INV-8). SLICE-4 makes this
-// button adaptive (Edit Selection when text is selected in the preview).
+// F11 (SLICE-4): the single adaptive Ask-Claude button. Its label flips on
+// `selectionchange` (Edit Selection when live text is selected in the preview,
+// Edit Document otherwise), and a click resolves the selection to a SOURCE range
+// via the nearest `data-src` ancestors (INV-36) — the webview's sole mapping
+// duty. A selection that resolves to no live block falls back to document scope.
+
+/** Walk up from a DOM node to the nearest block carrying data-src offsets (INV-36). */
+function nearestSrc(node: Node | null): HTMLElement | null {
+  let el: HTMLElement | null = node instanceof HTMLElement ? node : (node?.parentElement ?? null);
+  while (el && el !== body) {
+    if (el.dataset.srcStart !== undefined && el.dataset.srcEnd !== undefined) return el;
+    el = el.parentElement;
+  }
+  return null;
+}
+
+/** The source [start,end) union of the live blocks a non-empty body selection touches, or null. */
+function selectionSrcRange(): { start: number; end: number } | null {
+  const sel = window.getSelection();
+  if (!sel || sel.isCollapsed || sel.rangeCount === 0) return null;
+  const ends = [nearestSrc(sel.anchorNode), nearestSrc(sel.focusNode)].filter(
+    (e): e is HTMLElement => e !== null,
+  );
+  if (ends.length === 0) return null; // selection touches no live-source block
+  const starts = ends.map((e) => Number(e.dataset.srcStart));
+  const stops = ends.map((e) => Number(e.dataset.srcEnd));
+  return { start: Math.min(...starts), end: Math.max(...stops) };
+}
+
+function updateAskLabel(): void {
+  if (!askEl) return;
+  askEl.textContent = selectionSrcRange()
+    ? "✦ Ask Claude to Edit Selection"
+    : "✦ Ask Claude to Edit Document";
+}
+
+document.addEventListener("selectionchange", updateAskLabel);
+
 askEl?.addEventListener("click", () => {
-  vscodeApi.postMessage({ type: "askClaude", scope: "document" });
+  const range = selectionSrcRange();
+  if (range) {
+    vscodeApi.postMessage({ type: "askClaude", scope: "selection", start: range.start, end: range.end });
+  } else {
+    vscodeApi.postMessage({ type: "askClaude", scope: "document" });
+  }
 });
 
 // F10: delegated ✓/✗ accept/reject of pending proposals (routed back to the F4 seam).
@@ -83,6 +123,7 @@ window.addEventListener("message", (event: MessageEvent<RenderMessage>) => {
   const msg = event.data;
   if (msg?.type !== "render") return;
   body.innerHTML = msg.html;
+  updateAskLabel(); // new content clears any selection → reset the adaptive label
   const on = msg.mode === "on";
   if (annotationsEl) annotationsEl.checked = on;
   // Off-state is a clean preview: hide the review chrome.