fix(e24): the Home surface's three UAT findings — Back, the reroll, and a show card

All three were measured before they were touched, and the diagnosis moved twice.

**Back from a Plex title played off Home landed on the Plex module's root.** One
click pushed three history entries: the page navigation, the sub-view opened by
the `jumpTo` hand-off, and — under StrictMode, whose effect body runs twice
before `setJumpTo(null)` re-renders — that same sub-view a second time. So the
first Back visibly did nothing and the second stranded the user on a Discover
page they had never visited.

A cross-module hand-off is the CONTINUATION of the navigation that brought you
there, not a second navigation, so it now REPLACES the page entry instead of
stacking on it (`useHistorySubview.open(view, {replace})`, asked for by the jump
target). Replacing is also idempotent, which is what disarms the StrictMode
double-run. The intent rides on `PlexJumpTarget` rather than being inferred,
because the header typeahead jumps from INSIDE the module, where the entry being
replaced would be a sub-view the user really was looking at.

**"Mást kérek" cycled nine cards and never left the resume tier.** Measured:
twelve presses, nine distinct titles, period nine, every one `tier: "resume"` —
the shelf and the surprise tier were unreachable by pressing the button. Two
causes multiplied: the tiers were tried in a fixed priority order over a 20-row
resume rail, so the button just walked down it; and the client's rolling
exclusion window of 8 guaranteed the wrap.

The press count now rotates which tier answers first (`start`), so each press
offers a different KIND of answer — the user's call, given the choice between
that, a wider window, and a surprise-only reroll. The fallthrough is kept, so a
press does not come back empty because the tier it led with had nothing. The
window is matched to the server's own cap rather than a smaller number of ours.

**A half-watched series on Home opened the show page.** A movie already went
straight to the player; only shows did not — and the comfort shelf and the hero
both resolved a show to its next episode while this rail alone did not, handing
back six seasons to choose between. The rail row now carries `next`, and the
client already played `next ?? card`, so this is a backend fix.

Affording it needed `queries.next_episode` to stop loading every episode of the
show into Python to pick one of them (236 rows for Friends, 227 for JAG) — three
ordered LIMIT 1 probes instead. The show→episode rule now lives in ONE function
(`rails.playable`), which is what the shelf, the hero and the rail all call; two
implementations are how they came to disagree.

Ten mutations run, all ten red for the right test — including a first pass where
the press count reached `api.discoverPick` and stopped there, invisible because
every Home test mocks that module. The slice now has its own URL test.

Verified in a clean browser tab against the live account: one Back returns to
Home, the rotation gives resume/comfort/surprise across nine presses with no
repeat, and Westworld opens at S1E1 rather than at its show page.

frontend/src/components/PlexProvider.tsx

@@ -33,7 +33,17 @@ import { useNavigation } from "./NavigationProvider";
 // single Play button — the PLAYER for a movie/episode rating_key. `play` exists because the player
 // lives inside this module with its lazy hls.js and its provider tree; mounting a second one on the
 // landing page to avoid one hand-off would be the expensive way round.
-export type PlexJumpTarget = { id: string; type: "movie" | "show" | "play" };
+export type PlexJumpTarget = {
+  id: string;
+  type: "movie" | "show" | "play";
+  /** The jump ARRIVED WITH a page change — another module set this and sent the user here in one
+   *  gesture (the Home surface plays a Plex title). The opened sub-view then REPLACES the page entry
+   *  that hand-off just pushed, instead of stacking on top of it, so Back returns to where the user
+   *  actually came from rather than to a module root they never visited. Omitted for a jump made
+   *  from INSIDE the Plex module (the header typeahead), where the entry being replaced would be a
+   *  sub-view the user really was looking at. */
+  replace?: boolean;
+};
 
 type PlexState = {
   scope: string; // movie | show | both (unified cross-library scope)