feat(keys): j/k and the arrows walk Home's rails and every Plex grid and rail

Second slice of the E11 keyboard layer (S4b). S4a gave the feed, the library and the channel page
`j`/`k`, the arrows and Enter; Home's shelves and the whole Plex module were still Tab-stop country —
a Plex poster is its own Tab stop, and a rail's cards sat behind every control of the cards before.

- `lib/listNav`: a list that mounts every item no longer declares a count or columns — both are
  measured off the DOM when a key is pressed, so an auto-fill grid's columns follow its width and a
  rail is one row. ↑ on a list's top row and ↓ on its bottom row go on to the neighbouring list, onto
  the item horizontally nearest the one left (owner decision D4: ←→ within a rail, ↑↓ to the rail
  above or below). The first `j` skips cards a rail has scrolled out to the left.
- `Carousel keyboardNav` registers the scroller; the scroller gets a 4px inset (against a -4px margin,
  layout unchanged) so the focus ring is not clipped, and `[data-nav-index]` a 2.5rem inline scroll
  margin so a card stepped into view clears the rail's edge fade and paging arrow.
- `ui/NavGrid`: the plain, fully mounted grid that registers itself.
- `navIndex` on PlexPosterCard / SeasonCard / EpisodeCard (the card root is the item, the poster the
  open element — the ring goes round poster and title, D2), PosterTile and Home's ShelfTile (the button
  is the item); cast, related and collection-strip buttons carry the attrs directly.
- Wired: Home's comfort and resume rails; Plex Browse titles + episodes, Discover rails + genres,
  collection view, playlists/collections browse, person known-for + filmography, show seasons, season
  episodes, cast/related strips, info-page cast + collection strips.
- `listNav.wiring.contract.test`: every `Carousel` says `keyboardNav`, every card says `navIndex`, and a
  card with `navIndex` sits inside a NavGrid / Carousel / VirtualGrid — both halves are opt-in, and
  either one missing leaves a list the keyboard silently skips.

CLAUDE.md

@@ -90,7 +90,7 @@ CQRS, a repository/DI framework, a router library, a generic CRUD-router factory
 | FE per-account state    | `usePref`                                                    | raw `readAccount`/`writeAccount` or bespoke hooks      |
 | FE action button        | `ui/Button` — pick a `SIZES` rung                            | a hand-rolled `<button className="px-3 py-2 …">`       |
 | FE keyboard shortcut    | a row in `lib/keymap.ts` + `dispatch()`                      | a private `keydown` listener with its own key literals |
-| FE list/grid keys       | `useListNav` + `navItemAttrs(i)` + one `data-nav-open`       | a private arrow-key / roving-tabindex handler          |
+| FE list/grid keys       | `NavGrid`/`keyboardNav` + `navItemAttrs` + `data-nav-open`   | a private arrow-key / roving-tabindex handler          |
 | FE control              | the chip/segmented primitives (`Select` only for >6 options) | a native `<select>`                                    |
 | FE loading/empty/error  | `QueryState` / `StateMessage`                                | ad-hoc conditionals                                    |
 | FE failed write         | `meta.failTitle`, else `notifyActionError(err, title)`       | a bare `catch {}` or `notify()` with no error attached |