fix(memory): address PR review — numpy/UPSERT soft deps + BM25 floor + BLOB dim

- numpy soft dependency: try/except import + _HAS_NUMPY flag; _encode_embedding
  and _decode_embedding fall back to struct.pack/unpack; search_vector falls back
  to pure-Python cosine loop — startup never fails without numpy reinstalled
- SQLite UPSERT guard: _HAS_UPSERT = sqlite_version_info >= (3,24,0); save_chunk
  and save_chunks_batch fall back to INSERT OR REPLACE on SQLite < 3.24 with a
  one-time startup warning about potential FTS rowid drift
- _bm25_rank_to_score floor: 0.3 + 0.69*(|rank|/(1+|rank|)) → always in [0.3, 0.99),
  prevents small-corpus matches scoring 0.0 and being filtered by min_score
- detect_index_dim BLOB-aware: check isinstance(raw, bytes) first and return
  len(raw)//4 before json.loads, so /memory status works after embedding format switch
- Comment: "CJK single-char" → "CJK tokens shorter than 3 characters"

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

agent/memory/embedding/state.py

@@ -31,9 +31,13 @@ def detect_index_dim(storage) -> Optional[int]:
     if not row or not row["embedding"]:
         return None
     try:
-        emb = json.loads(row["embedding"])
+        raw = row["embedding"]
+        if isinstance(raw, (bytes, bytearray)):
+            # New BLOB format: 4 bytes per float32
+            return len(raw) // 4
+        emb = json.loads(raw)
         return len(emb) if isinstance(emb, list) else None
-    except (json.JSONDecodeError, TypeError):
+    except (json.JSONDecodeError, TypeError, Exception):
         return None