perf(series): the episode list no longer waits on the server

Opening Frasier on a Fairphone took ~5 s to render the episode list
although every episode was cached. Three causes:

- resolve_series_view waited for Next Up and resume before returning the
  episodes, and Next Up was server-first. The episode list now returns as
  soon as the episodes are in (with_hints); hints that have answered are
  used, late ones dropped, and the picker falls back to local watch state.
  Next Up is cache-first like every other query.
- The page loaded itself six times per open: onMount plus a mount-time
  $effect, the reachability effect's first run posing as a reconnect, and
  a double mount. All triggers now share one coalesced load per item
  (createCoalescedLoader); refresh triggers get one re-run after it.
- The root layout rendered the route in two branches that each rendered
  children; the page store deciding between them updates a flush late, so
  navigating Search -> library page mounted the page twice. One element
  now renders the route and only its classes change.

On the device: one load per open, seasons from cache in 14 ms, episodes
and the Resume button up in under a second (was ~5 s).

docs/architecture/02-svelte-frontend.md

@@ -666,6 +666,14 @@ render behind it. There is no measurement and no reserved padding. If you
 restructure the shell, preserve the scroll containment — reintroducing padding
 math reintroduces the bug.
 
+**The route renders in exactly one element.** `+layout.svelte` switches the
+wrapper's *classes* between the shell scroller and the plain clipped box that
+layout-owning routes (library, settings, player) get — it must not switch
+between two branches that each render `children`. The page store that decides
+the mode can update a flush after the new route renders, so two branches
+mounted a page under one and then remounted it under the other: every
+navigation between the two kinds of route loaded the page twice (DR-295).
+
 ### AccountMenu
 
 One component for both breakpoints, anchored to the username/avatar (a real
@@ -719,6 +727,22 @@ hero button labelled `Resume S2E4` / `Play S1E1`.
 A season is not a destination: `/library/<seasonId>` redirects to its series
 (DR-103). Video library routes collapse to one per library (DR-105).
 
+**The episode list never waits for Next Up or resume.** `resolve_series_view`
+(`series_progress.rs`, `with_hints`) returns as soon as the episodes are in;
+Next Up and resume are used if they have answered by then and dropped if not,
+and `pick_current_episode` falls back to the episodes' own watch state. They
+only refine which episode is current, and waiting for them held the list for
+the server's 2–3 s although every episode was cached. Next Up is cache-first
+like every other query (03-data-flow).
+
+**One load per item, however many triggers.** The detail page loads through
+`createCoalescedLoader` (`utils/coalescedLoader.ts`): calls for the item
+already loading share that load, and callers that know the data changed
+(`fresh`: reconnect, filter change, mark watched, clear history) get exactly
+one re-run after it. `onMount`, a mount-time `$effect`, the reachability
+effect's first run and the double mount above used to each start a full load —
+six per open, about seventy requests in flight.
+
 `episodeStrip.ts` holds the pure logic for the "More Episodes" strip, extracted
 from the component because it had three distinct bugs that markup made
 untestable: the strip collapsing to just the current episode while real siblings