feat: add boost directive to FeedEnhancement

Post-processors can now return a boost map (item ID -> score)
to promote or demote items in the feed ordering. Scores from
multiple processors are summed and clamped to [-1, 1].

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

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

@@ -14,6 +14,8 @@ export interface FeedEnhancement {
 	groupedItems?: ItemGroup[]
 	/** Item IDs to remove from the feed */
 	suppress?: string[]
+	/** Map of item ID to boost score (-1 to 1). Positive promotes, negative demotes. */
+	boost?: Record<string, number>
 }
 
 /**