refactor: use reduce for stale group filtering

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

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

@@ -306,6 +306,10 @@ export class FeedEngine<TItems extends FeedItem = FeedItem> {
 				const enhancement = await processor(currentItems)
 
 				if (enhancement.additionalItems?.length) {
+					// Post-processors operate on FeedItem[] without knowledge of TItems.
+					// Additional items are merged untyped — this is intentional. The
+					// processor contract is "FeedItem in, FeedItem out"; type narrowing
+					// is the caller's responsibility when consuming FeedResult.
 					currentItems = [...currentItems, ...(enhancement.additionalItems as TItems[])]
 				}
 
@@ -327,7 +331,17 @@ 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.reduce<ItemGroup[]>((acc, group) => {
+			const ids = group.itemIds.filter((id) => itemIds.has(id))
+			if (ids.length > 0) {
+				acc.push({ ...group, itemIds: ids })
+			}
+			return acc
+		}, [])
+
+		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)]))