feat: add post-processor pipeline to FeedEngine (#41)

* feat: add post-processor pipeline to FeedEngine

Add FeedPostProcessor type and FeedEnhancement interface.
Post-processors run after item collection on all update
paths (refresh, reactive context, reactive items).

Pipeline is chained — each processor sees items as modified
by the previous one. Enhancement merging handles additional
items, suppression, and grouped items. Throwing processors
are caught and recorded in FeedResult.errors.

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

* docs: document intentional TItems cast in post-processor merge

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

* 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>

* refactor: use reduce for stale group filtering

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

---------

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

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

@@ -0,0 +1,23 @@
+import type { FeedItem } from "./feed"
+
+export interface ItemGroup {
+	/** IDs of items to present together */
+	itemIds: string[]
+	/** Summary text for the group */
+	summary: string
+}
+
+export interface FeedEnhancement {
+	/** New items to inject into the feed */
+	additionalItems?: FeedItem[]
+	/** Groups of items to present together with a summary */
+	groupedItems?: ItemGroup[]
+	/** Item IDs to remove from the feed */
+	suppress?: string[]
+}
+
+/**
+ * A function that transforms feed items and produces enhancement directives.
+ * Use named functions for meaningful error attribution.
+ */
+export type FeedPostProcessor = (items: FeedItem[]) => Promise<FeedEnhancement>