fix(library): populate "More Episodes" for series without season folders

The Episode Focus View's episode strip collapsed to just the current
episode on some series. Two causes:

- Series that expose episodes directly as children rather than under
  season folders yielded an empty season fetch, leaving allEpisodes
  empty. The library page now groups those flat episode children by
  their season number and synthesizes minimal season headers.
- isCurrentEpisode over-matched: episodes with no season/episode number
  compared equal (undefined === undefined) and every one of them looked
  like the focused episode.

Extracts the strip's pure logic into episodeStrip.ts so both behaviours
are unit-tested, per the failing-test-first rule.

TRACES: UR-058 | DR-087

src/routes/library/[id]/+page.svelte

@@ -147,6 +147,34 @@
         // Sort seasons by index number
         seasonData.sort((a, b) => (a.season.indexNumber || 0) - (b.season.indexNumber || 0));
 
+        // Some series expose episodes directly as children rather than under
+        // season folders. In that case the season fetch above yields nothing —
+        // group the flat episode children by their season number so the Episode
+        // Focus View still has a populated `allEpisodes` (otherwise "More
+        // Episodes" collapses to just the current episode).
+        if (seasonData.every((s) => s.episodes.length === 0)) {
+          const flatEpisodes = $libraryItems.filter((i) => i.kind === "episode");
+          if (flatEpisodes.length > 0) {
+            const bySeason = new Map<number, MediaItem[]>();
+            for (const ep of flatEpisodes) {
+              const key = ep.parentIndexNumber ?? 1;
+              (bySeason.get(key) ?? bySeason.set(key, []).get(key)!).push(ep);
+            }
+            seasonData = [...bySeason.entries()]
+              .sort(([a], [b]) => a - b)
+              .map(([seasonNumber, episodes]) => ({
+                // Synthesize a minimal season header from the episodes we have.
+                season: {
+                  ...(seasons.find((s) => s.indexNumber === seasonNumber) ?? episodes[0]),
+                  kind: "season",
+                  indexNumber: seasonNumber,
+                  name: `Season ${seasonNumber}`,
+                } as MediaItem,
+                episodes: episodes.sort((a, b) => (a.indexNumber || 0) - (b.indexNumber || 0)),
+              }));
+          }
+        }
+
         // If we have a focused episode ID but couldn't find it in the seasons,
         // fetch it directly (handles ID mismatch between APIs)
         const episodeIdParam = $page.url.searchParams.get("episode");