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/embedding/chunk_ids.rs

@@ -14,6 +14,10 @@ pub fn encode_rowid(document_id: i64, chunk_index: i64) -> i64 {
 }
 
 pub fn decode_rowid(rowid: i64) -> (i64, i64) {
+    debug_assert!(
+        rowid >= 0,
+        "decode_rowid called with negative rowid: {rowid}"
+    );
     let document_id = rowid / CHUNK_ROWID_MULTIPLIER;
     let chunk_index = rowid % CHUNK_ROWID_MULTIPLIER;
     (document_id, chunk_index)