Apply code review feedback to client refactor

- Rename phase_time to step_time for consistency with Step enum
- Unify blob_size and file_size into single blob_size field
- Merge toastyfs_alloc into toastyfs_init, returning malloced ToastyFS*
- Replace (MessageHeader*)&msg casts with &msg.base
- Replace !resp.size with explicit resp.size == 0

https://claude.ai/code/session_018MRQTdAZoBCXJZgdP4U6V6

src/random_client.c

@@ -39,12 +39,9 @@ int random_client_init(void *state_, int argc, char **argv,
         }
     }
 
-    state->tfs = toastyfs_alloc();
-    if (state->tfs == NULL)
-        return -1;
-
     uint64_t client_id = next_random_client_id++;
-    if (toastyfs_init(state->tfs, client_id, addrs, num_addrs) < 0)
+    state->tfs = toastyfs_init(client_id, addrs, num_addrs);
+    if (state->tfs == NULL)
         return -1;
 
     *timeout = 0;