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).

src/routes/+layout.svelte

@@ -73,7 +73,8 @@
   // a new page inherits the previous page's offset. Must be registered here at
   // init, alongside the tracker above, for the same reason. (DR-156)
   let shellScroller = $state<HTMLElement>();
-  useScrollRestore(() => shellScroller, "shell");
+  // Owned routes scroll inside their own column; the shell box does not.
+  useScrollRestore(() => (routeOwnsLayout ? undefined : shellScroller), "shell");
 
   // Layout-shell visibility rules live in one pure, unit-tested module
   // ($lib/utils/layoutShell) so they can't drift per route/platform.
@@ -357,29 +358,29 @@
          scrolling internally. All other top-level pages render directly here, so
          this wrapper must scroll and reserve the fixed bottom UI's measured
          height so the mini player / bottom nav never overlap the last rows. -->
-    {#if routeOwnsLayout}
-      <!-- These routes own their own full-height flex column (header + scroller
-           + their own in-flow BottomUi), so the root just clips and steps back. -->
-      <div class="flex-1 overflow-hidden">
-        {@render children()}
-      </div>
-    {:else}
-      <!-- Shared header (account menu, desktop nav) as a flex-shrink-0 sibling
-           above the scroller, so it never eats into the scroller's bounds. -->
-      {#if showGlobalHeader}
-        <AppHeader />
-      {/if}
-      <!-- Scroller is flex-1/min-h-0; the in-flow BottomUi below is a flex
-           sibling, so the list is physically bounded above it and can never
-           render behind it. No measurement, no reserved padding. -->
-      <div
-        bind:this={shellScroller}
-        class="flex-1 overflow-y-auto min-h-0"
-        style="overscroll-behavior: contain"
-      >
-        {@render children()}
-      </div>
+    <!-- Shared header (account menu, desktop nav) as a flex-shrink-0 sibling
+         above the scroller, so it never eats into the scroller's bounds. -->
+    {#if !routeOwnsLayout && showGlobalHeader}
+      <AppHeader />
     {/if}
+    <!-- ONE element renders the route, whatever the layout mode; only its
+         classes change. Routes that own their full-height column (header +
+         scroller + their own in-flow BottomUi) get a plain clipped box; every
+         other route gets the shell scroller (flex-1/min-h-0, bounded above the
+         in-flow BottomUi, so no measurement or reserved padding).
+
+         This used to be two branches, each rendering `children`. The page
+         store that decides the mode can update a flush after the new route
+         renders, so navigating between the two kinds of route (Search → a
+         library page) mounted the page under one branch and then *remounted*
+         it under the other — every load it started, twice (DR-295). -->
+    <div
+      bind:this={shellScroller}
+      class={routeOwnsLayout ? "flex-1 overflow-hidden" : "flex-1 overflow-y-auto min-h-0"}
+      style={routeOwnsLayout ? undefined : "overscroll-behavior: contain"}
+    >
+      {@render children()}
+    </div>
 
     <!-- Re-authentication modal -->
     <ReauthModal isOpen={$needsReauth} />