Indexes on view outputs (VV3) #325

Part of epic #305.

Problem. .index create accepts a rule name with a schema and builds an empty, never-maintained index (src/storage_engine/vector_index.rs:57-146); hnsw_nearest over a derived relation is impossible.

Target. .index create <name> on <view>(<column>) for a deployed rule whose first column is an integer id; the view's output batches drive the index (+1 insert, -1 tombstone) at the batch's revision; the index is dropped and rebuilt with the view's generation (V7) and parked with the KG; .index list shows the source kind.

Acceptance. Index on a view equals the view's rows at every revision, including through a rule replacement (generation swap) and a base-relation delete that empties part of the view; definition persisted and rebuilt at restart; refusal for views whose first column is not an integer id.

Size M. Dependencies V7, VV2. Order slot 7.