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/index.ts

@@ -13,6 +13,9 @@ export { TimeRelevance } from "./feed"
 // Feed Source
 export type { FeedSource } from "./feed-source"
 
+// Feed Post-Processor
+export type { FeedEnhancement, FeedPostProcessor, ItemGroup } from "./feed-post-processor"
+
 // Feed Engine
 export type { FeedEngineConfig, FeedResult, FeedSubscriber, SourceError } from "./feed-engine"
 export { FeedEngine } from "./feed-engine"