feat(library): Filters shows its active count; final review follow-ups

- D44: the shared toolbar's Filters button reads "Filters · N" while N
  "Refine this view" filters are active (Library: Tags; Images: Source,
  Folder, Media, Tags), so a Browse-set Tags filter is visible on the
  shelf. The text is the accessible name. Unit and browser tests cover
  Browse -> "Filters · 1" -> Clear -> "Filters" and the Images counts.
- Renumber the alphabetical tag chips deviation D42 -> D43 (D42 is #264's).
- The caption count is a role=status region written only when it changes
  and kept shown while the shelf reloads.
- usedLabel names the year for a use in another calendar year (D23).
- prompt-blocks: same `title` option API as #264 (`subject` suffix); the
  region-name helper's parameter no longer shadows it.
- Scope library.css .row-actions rules to the history panels; fix stale
  CSS and spec comments.
- Retry re-reading a failed category list and the no-op Apply/search
  paths are covered by a browser test.
- TESTING.md: D23 amendments (year, button vs link trade-off), D44, the
  legacy Library link wording.

tests/js/workspace-library-format.test.js

@@ -23,6 +23,9 @@ test('used label boundaries in UTC and local time', async () => {
         assert.equal(usedLabel('2026-10-14T00:00:00Z', now), 'Used 6 days ago');
         assert.equal(usedLabel('2026-10-13T23:59:59Z', now), 'Used Oct 13');
         assert.equal(usedLabel('2026-01-02T08:00:00Z', now), 'Used Jan 2');
+        // D23: a use in another calendar year names its year, so last year's Oct 21 never reads as this year's.
+        assert.equal(usedLabel('2025-10-21T08:00:00Z', now), 'Used Oct 21, 2025');
+        assert.equal(usedLabel('2025-12-31T23:59:59Z', now), 'Used Dec 31, 2025');
         // A clock ahead of this browser is not a future use: it reads as today.
         assert.equal(usedLabel('2026-10-21T09:00:00Z', now), 'Used today');
     });