fix(search): cap vector search k-value and add rowid assertion

The vector search multiplier could grow unbounded on documents with
many chunks, producing enormous k values that cause SQLite to scan
far more rows than necessary. Clamp the multiplier to [8, 200] and
cap k at 10,000 to prevent degenerate performance on large corpora.

Also adds a debug_assert in decode_rowid to catch negative rowids
early — these indicate a bug in the encoding pipeline and should
fail fast rather than silently produce garbage document IDs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/search/vector.rs

@@ -51,8 +51,8 @@ pub fn search_vector(
         .collect();
 
     let max_chunks = max_chunks_per_document(conn).max(1);
-    let multiplier = ((max_chunks.unsigned_abs() as usize * 3 / 2) + 1).max(8);
-    let k = limit * multiplier;
+    let multiplier = ((max_chunks.unsigned_abs() as usize * 3 / 2) + 1).clamp(8, 200);
+    let k = (limit * multiplier).min(10_000);
 
     let mut stmt = conn.prepare(
         "SELECT rowid, distance