test/docs(404): add unrelated-edit round-trip test; correct MultiCollection-only rationale

Review + live-E2E follow-ups (no behavior change):
- Add a test proving the canonical silent-reset trap directly: a rename-only save
  round-trips both weights untouched (the prior test only edited the weight it
  asserted). Cold review nit.
- Correct the rationale in itemRules.ts + decisions.md: WeightedShuffle is
  MultiCollection-only in the SPA for *meaningfulness* (per-source weights need
  2+ sources), NOT because the classic write path rejects it — live-E2E confirmed
  the classic engine ACCEPTS it on a plain Collection (200) and degrades to
  fair-share. The rejection is on the separate playlist/block write paths, whose
  editors keep their own order lists and already omit it.

Live-E2E (real API): weighted multi-collection create + read round-trips weights;
rename-only PUT preserves them (no silent reset); WeightedShuffle persists on a
classic MultiCollection schedule item. Ratio itself is pinned by the existing
PlayoutBuildGoldenTests.Classic_weighted (3:1), untouched by this SPA change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/decisions.md

@@ -1956,10 +1956,13 @@ is purely SPA (+ docs).
   classic schedule item's Playback Order options. Putting weights on the schedule item was rejected: the same
   multi-collection can be referenced by many schedule items, and weights are shared, not per-reference.
 - **`WeightedShuffle` is offered ONLY for `collectionType === 'MultiCollection'`** (`itemRules.ts`
-  `MULTI_COLLECTION_ORDERS`). It needs per-source weights, which only exist on multi-collection members, so it
-  is meaningless on the single-source collection types — and the classic write path *rejects* it for
-  playlist/block items (decisions.md 2026-07-17), so offering it there would only produce a 400. This is the
-  SPA mirror of that write-path gate.
+  `MULTI_COLLECTION_ORDERS`) — a *meaningfulness* scope, not a rejection mirror. Weights only exist on
+  multi-collection members, so the order only does something with 2+ weighted sources; on a single collection
+  the classic engine still **accepts** it (verified by live-E2E: `PUT /schedules/{id}/items` with a plain
+  Collection + `WeightedShuffle` → 200) but it degrades to fair-share (≈ `Shuffle`), a confusing no-op — so
+  the SPA doesn't offer it there. The *rejection* the #70 entry describes is on the separate **playlist and
+  block** write paths (different engines); the playlist/block editors keep their own order lists (not
+  `MULTI_COLLECTION_ORDERS`) and already omit `WeightedShuffle`, so nothing extra was needed there.
 - **`WeightedShuffle` joins `ShuffleInOrder` in the `fillWithGroup` exclusion.** `fillWithGroupModeEligible`
   already excluded `ShuffleInOrder`; WeightedShuffle is excluded for the same structural reason —
   `PlayoutBuilder` splits a fill-with-group item into per-group enumerators scheduled one group at a time,

web/src/schedules/itemRules.ts

@@ -43,10 +43,12 @@ export function nextDraftKey(): string {
 
 // ---- Option lists --------------------------------------------------------
 
-// WeightedShuffle (#70) is offered ONLY here: it needs per-source weights, which live on
-// MultiCollectionItem/SmartItem, so it is meaningless on the single-source collection types. The
-// classic write path rejects it everywhere else (playlist/block items), so the SPA must not offer it
-// there either (docs/decisions.md 2026-07-17).
+// WeightedShuffle (#70) is offered ONLY for MultiCollection: it distributes airtime across per-source
+// weights, which live on MultiCollectionItem/SmartItem, so it is only *meaningful* with 2+ weighted
+// sources. On a single collection the classic engine still accepts it but it degrades to fair-share
+// (≈ Shuffle), a confusing no-op — hence MultiCollection-only here. (Distinct concern: the playlist and
+// block editors — which keep their own order lists, not this one — must also omit it because those
+// write paths reject it outright; docs/decisions.md 2026-07-17.)
 const MULTI_COLLECTION_ORDERS: PlaybackOrder[] = ['Shuffle', 'ShuffleInOrder', 'WeightedShuffle'];
 const COLLECTION_LIKE_ORDERS: PlaybackOrder[] = ['Chronological', 'Random', 'Shuffle', 'ShuffleInOrder', 'Marathon'];
 const TV_SHOW_ORDERS: PlaybackOrder[] = ['Chronological', 'SeasonEpisode', 'Random', 'Shuffle', 'MultiEpisodeShuffle'];

web/src/screens/MultiCollectionsScreen.test.tsx

@@ -162,20 +162,60 @@ describe('MultiCollectionsScreen', () => {
     });
   });
 
+  // Weighted multi-collection fixture reused by the round-trip tests: 3:1 split (Favorites : Action).
+  const weightedMulti = [
+    {
+      id: 1,
+      items: [
+        { collectionId: 5, name: 'Favorites', playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 3 },
+        { collectionId: null, name: 'Action', playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
+      ],
+      name: 'Saturday Bundle'
+    }
+  ];
+
+  it('preserves weights when only an unrelated field is edited (the silent-reset trap)', async () => {
+    // The canonical #404 trap: the replace-all PUT resubmits the whole item list, so editing ONLY the
+    // name must still round-trip each source's weight untouched — dropping it in the draft mapping
+    // would silently reset every weight to 1 on this save (docs/decisions.md 2026-07-17).
+    const fetchMock = mockApi({
+      multi: weightedMulti,
+      onRequest: (url, method) =>
+        url === '/api/v1/multi-collections/1' && method === 'PUT'
+          ? jsonResponse({ id: 1, items: [], name: 'Weekend Bundle' })
+          : null
+    });
+
+    render(<MultiCollectionsScreen />);
+    fireEvent.click(await screen.findByText('Saturday Bundle'));
+    await screen.findByLabelText('Weight for Favorites');
+
+    // Edit ONLY the name — never touch a weight input.
+    fireEvent.change(screen.getByPlaceholderText('Multi-collection name'), { target: { value: 'Weekend Bundle' } });
+    fireEvent.click(screen.getByRole('button', { name: 'Save multi-collection' }));
+
+    await waitFor(() => {
+      const putCall = fetchMock.mock.calls.find(
+        ([u, init]) => u === '/api/v1/multi-collections/1' && (init?.method ?? '').toUpperCase() === 'PUT'
+      );
+      expect(putCall).toBeDefined();
+      const parsed = JSON.parse(String(putCall?.[1]?.body));
+      expect(parsed.name).toBe('Weekend Bundle');
+      // Both original weights survive unchanged — NOT reset to 1.
+      expect(parsed.items).toEqual(
+        expect.arrayContaining([
+          { collectionId: 5, playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 3 },
+          { collectionId: null, playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
+        ])
+      );
+    });
+  });
+
   it('round-trips per-source weights on save (no silent reset), and edits them', async () => {
     // Weighted multi-collection: the API returns each item's weight and the update REPLACES the item
     // list, so the editor must carry weight through untouched or every source silently resets to 1
     // (docs/decisions.md 2026-07-17, #404).
-    const weighted = [
-      {
-        id: 1,
-        items: [
-          { collectionId: 5, name: 'Favorites', playbackOrder: 'Chronological', scheduleAsGroup: false, smartCollectionId: null, weight: 3 },
-          { collectionId: null, name: 'Action', playbackOrder: 'Chronological', scheduleAsGroup: true, smartCollectionId: 9, weight: 1 }
-        ],
-        name: 'Saturday Bundle'
-      }
-    ];
+    const weighted = weightedMulti;
 
     const fetchMock = mockApi({
       multi: weighted,