Byte quota enforcement

Track space usage under a byte quota. A store maps string keys to blob sizes (bytes) and enforces a fixed quota on total bytes used. Process operations: ["write", key, size] attempts to store/overwrite key with a blob of size bytes, it succeeds only if total used bytes AFTER the write would be <= quota (when overwriting an existing key, the old size is released first for this check); on failure the write is rejected and the key keeps its previous value (or stays absent). ["delete", key] removes key and frees its bytes (no-op if absent). Return a list of booleans, one per "write" op in order, indicating whether each write succeeded. size >= 0; <= 10^5 ops.