feat(search): add score_kind to VectorRetriever (Cosine) and hybrid test helpers (Rrf)

This commit unblocks Tasks 3 and 4 of fb-38:
- VectorRetriever::build_hit now labels hits with ScoreKind::Cosine
- Hybrid retriever test helpers (mk_hit functions) label synthetic hits with ScoreKind::Rrf
- Updated lexical snapshot fixture to reflect new score_kind field in output

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-search/src/hybrid.rs

@@ -505,6 +505,7 @@ mod tests {
             // a fixed UNIX_EPOCH so synthetic hits remain deterministic.
             indexed_at: time::OffsetDateTime::UNIX_EPOCH,
             stale: false,
+            score_kind: kebab_core::ScoreKind::Rrf,
         }
     }
 
@@ -755,6 +756,7 @@ mod tests {
                 chunker_version: ChunkerVersion("c1".into()),
                 indexed_at: time::OffsetDateTime::UNIX_EPOCH,
                 stale: false,
+                score_kind: kebab_core::ScoreKind::Rrf,
             }
         }
 

crates/kebab-search/src/vector.rs

@@ -21,7 +21,7 @@ use std::sync::Arc;
 use anyhow::{Context, Result};
 use kebab_core::{
     ChunkId, ChunkerVersion, DocumentId, Embedder, EmbeddingInput, EmbeddingKind,
-    IndexVersion, RetrievalDetail, Retriever, SearchHit, SearchMode, SearchQuery,
+    IndexVersion, RetrievalDetail, Retriever, ScoreKind, SearchHit, SearchMode, SearchQuery,
     SourceSpan, VectorHit, VectorStore, WorkspacePath,
 };
 use kebab_store_sqlite::SqliteStore;
@@ -326,6 +326,7 @@ fn build_hit(
         // (called from `App::search` / `App::search_uncached`) and the equivalent
         // in `RagPipeline::ask` against the configured threshold.
         stale: false,
+        score_kind: ScoreKind::Cosine,
     })
 }
 

crates/kebab-search/tests/fixtures/search/lexical/run-1.json

@@ -26,6 +26,7 @@
       "vector_rank": null,
       "vector_score": null
     },
+    "score_kind": "bm25",
     "section_label": "Snap",
     "snippet": "alpha alpha",
     "stale": false
@@ -57,6 +58,7 @@
       "vector_rank": null,
       "vector_score": null
     },
+    "score_kind": "bm25",
     "section_label": "Snap",
     "snippet": "alpha bravo charlie",
     "stale": false