fix: filter stale item IDs from groups after pipeline

Groups accumulated during the pipeline can reference items
that a later processor suppressed. The engine now strips
stale IDs and drops empty groups before returning.

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-core/src/feed-engine.ts

@@ -331,7 +331,16 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 			}
 		}
 
-		return { items: currentItems, groupedItems: allGroupedItems, errors: allErrors }
+		// Remove stale item IDs from groups and drop empty groups
+		const itemIds = new Set(currentItems.map((item) => item.id))
+		const validGroups = allGroupedItems
+			.map((group) => ({
+				...group,
+				itemIds: group.itemIds.filter((id) => itemIds.has(id)),
+			}))
+			.filter((group) => group.itemIds.length > 0)
+
+		return { items: currentItems, groupedItems: validGroups, errors: allErrors }
 	}
 
 	private ensureGraph(): SourceGraph {

packages/aris-core/src/feed-post-processor.test.ts

@@ -173,6 +173,32 @@ describe("FeedPostProcessor", () => {
 			])
 		})
 
+		test("stale item IDs are removed from groups after suppression", async () => {
+			const engine = new FeedEngine()
+				.register(
+					createCalendarSource([calendarItem("c1", "Meeting A"), calendarItem("c2", "Meeting B")]),
+				)
+				.registerPostProcessor(async () => ({
+					groupedItems: [{ itemIds: ["c1", "c2"], summary: "Afternoon" }],
+				}))
+				.registerPostProcessor(async () => ({ suppress: ["c1"] }))
+
+			const result = await engine.refresh()
+			expect(result.groupedItems).toEqual([{ itemIds: ["c2"], summary: "Afternoon" }])
+		})
+
+		test("groups with all items suppressed are dropped", async () => {
+			const engine = new FeedEngine()
+				.register(createCalendarSource([calendarItem("c1", "Meeting A")]))
+				.registerPostProcessor(async () => ({
+					groupedItems: [{ itemIds: ["c1"], summary: "Solo" }],
+				}))
+				.registerPostProcessor(async () => ({ suppress: ["c1"] }))
+
+			const result = await engine.refresh()
+			expect(result.groupedItems).toBeUndefined()
+		})
+
 		test("groupedItems is omitted when no processors produce groups", async () => {
 			const engine = new FeedEngine()
 				.register(createWeatherSource([weatherItem("w1", 20)]))