docs(filters): user manual — guide, cookbook, generated field reference

The filter manual (docs/user/filters/), folded in from the parked plan
now that the UI matches the backend (group ALL/ANY header toggle,
2344f568):

- index.md — the guide: the three homes of filters (drawer/saved
  filters, smart collections, dashboard sections), the books/episodes
  scope door, rule anatomy, value picking (chips with x, live counts
  under other rules), GROUPS via the new header toggle (+ the row
  chips as the fine-tuning tool), the live match count, today's
  defaults (reversed listening order, family-lens scoping,
  un-downloaded episodes), saving and shareable views.
- cookbook.md — seven exact-clicks recipes (born-reversed briefing,
  OR-across-authors, big-unread-fantasy, downloaded-unplayed,
  new-arrivals, dashboard shelf, shareable views). Every claim
  cross-checked against the generated reference — which caught the
  one wrong operator label in my first draft ("within last" vs the
  real "within the last"), the drift protection working as designed.
- fields.md — GENERATED from FilterFieldsPayload by
  TestFieldReferenceDocIsCurrent: the drift test fails the build if
  the registry ever changes without regenerating the docs (user
  decision: docs forced to match working code; FILTER_DOCS_REGEN=1
  regenerates).
- searching-and-filtering.md cross-links to the folder.

docs/user/searching-and-filtering.md

@@ -35,3 +35,12 @@ You can combine tags with other filters:
 - Tags use fuzzy matching - "Sci Fi" will match "Science Fiction"
 - Best matching tags appear first in results
 - Autocomplete shows existing tags as you type with relevance scores
+
+
+## The complete filter guide
+
+The filters described above are one system that also powers smart collections and dashboard
+sections — groups with ALL/ANY, saved filters, and every field listed. The full manual lives in
+[this folder](filters/index.md): the [guide](filters/index.md), the
+[cookbook](filters/cookbook.md) of ready-made recipes, and the generated
+[field reference](filters/fields.md).