test: close the last coverage debt the roadmap still listed

- `overflow-clip` gets a safelist anchor in index.css (its only occurrence is a `.ts` constant),
  and the chip-lane contract keeps it there.
- PlexPlayer's Backspace ownership rule is pinned on all three branches: nothing above, a foreign
  layer above (it defers), and its own panel open under a layer (it still closes the panel).
- Removing a playlist item bumps the list's `updated_at`, like the three other item-only writes.
- Seen in a real browser on localdev: an exhausted comfort-shelf tile (an empty pinned playlist) is
  focusable, aria-disabled, and its Shift+F10 menu offers Play off and Remove from shelf.

frontend/src/lib/chipLane.contract.test.ts

@@ -89,4 +89,12 @@ describe("chip lane containment contract", () => {
     // padding this canary, so it stayed satisfied with one real measurer fewer (round 2).
     expect(sources().filter(([, src]) => src.includes(MIRROR)).length).toBeGreaterThanOrEqual(3);
   });
+
+  // The clip is a Tailwind utility whose only occurrence is `chipLaneClass`, a `.ts` constant: if
+  // source detection ever stopped covering that file, the utility would leave the bundle and the
+  // leak above would return with every lane green. index.css anchors it; this keeps the anchor.
+  it("keeps the clip in the bundle whatever source detection sees", () => {
+    const css = readFileSync(new URL("../index.css", import.meta.url), "utf-8");
+    expect(css).toMatch(/@source inline\("[^"]*\boverflow-clip\b[^"]*"\)/);
+  });
 });