test(review): add the 4 new test-coverage points from PR #185 re-review

The re-review's blocking/structural points (lease leak, dup-id guard test,
body-before-title test, CHANGELOG, pg18, shared jsonb decoder) were already
addressed in commit 24264ef; this adds the 4 genuinely-new coverage requests:

- pt 6: `scrollToReference(id, index?)` exercised against a live editor DOM —
  selects the index-th `sup[data-footnote-ref][data-id]` occurrence, falls back
  to the first for out-of-range, returns false for an empty id (scrollIntoView
  stubbed). (#168)
- pt 7: export `backlinkLabel` and pin the base-26 carry boundary
  (25->z, 26->aa, 27->ab, 51->az, 52->ba). (#168)
- pt 8: integration fail-open — a PRESENT-but-corrupt tool_allowlist (jsonb
  string scalar holding non-array JSON) reads back as null ("no restriction"),
  covering normalizeRow's degrade branch. (#159 #172/#173)
- pt 9: getFootnoteRefCount cache invalidation — adding a `[^a]` reference bumps
  the cached count 2 -> 3. (#168)

Verified: editor-ext footnote 23; client structure 7 + tsc; server int 8.

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

apps/client/src/features/editor/components/footnote/footnote-definition-view.tsx

@@ -7,7 +7,7 @@ import classes from "./footnote.module.css";
  * A 0-based backlink index -> its lowercase letter label (0 -> "a", 25 -> "z",
  * 26 -> "aa", ...), matching the Pandoc/Wikipedia "↩ a b c" convention.
  */
-function backlinkLabel(index: number): string {
+export function backlinkLabel(index: number): string {
   let out = "";
   let x = index;
   while (x >= 0) {

apps/client/src/features/editor/components/footnote/footnote-views.structure.test.tsx

@@ -75,7 +75,9 @@ vi.mock("@/features/editor/components/code-block/mermaid-view.tsx", () => ({
 }));
 
 import FootnotesListView from "./footnotes-list-view";
-import FootnoteDefinitionView from "./footnote-definition-view";
+import FootnoteDefinitionView, {
+  backlinkLabel,
+} from "./footnote-definition-view";
 import CodeBlockView from "../code-block/code-block-view";
 
 // Minimal NodeViewProps stub: definition view only touches node.attrs.id and
@@ -214,3 +216,16 @@ describe("#168 footnote definition multi-backlinks", () => {
     );
   });
 });
+
+// #185 re-review pt 7: backlinkLabel is base-26 (a..z, then aa…). The component
+// tests only cover a,b,c (index 0-2); pin the >= 26 carry boundary.
+describe("backlinkLabel base-26 boundary (#168)", () => {
+  it("maps 0->a, 25->z, 26->aa, 27->ab, 51->az, 52->ba", () => {
+    expect(backlinkLabel(0)).toBe("a");
+    expect(backlinkLabel(25)).toBe("z");
+    expect(backlinkLabel(26)).toBe("aa");
+    expect(backlinkLabel(27)).toBe("ab");
+    expect(backlinkLabel(51)).toBe("az");
+    expect(backlinkLabel(52)).toBe("ba");
+  });
+});