SHM create race: attachers mmap a size-0 segment ("cannot mmap an empty file")

Symptom: consumers crash attaching to a SHM segment with ValueError: cannot mmap an empty file (seen on real runs, July sprint).

Race: stdlib SharedMemory(create=True, size=N) does shm_open then ftruncate. An attacher that opens in that window sees size 0 and mmaps nothing. _open_shm_with_retry (ipc_factory.py:49) only catches FileNotFoundError, so it never covered this. Affected paths are the name-derived symmetric ones: _create_or_open (:145) attach arm, plus direct SharedMemory(create=True, ...) sites (~:174).

Fix: explicit readiness, readers now wait until the segment actually has a size, with a 10s cap and a clear error if it never shows up. No sleep ladder, no blind retries.

Blast radius: shared transport infra — all robots, sim modules, cameras, every SHM pubsub user.

Start in dimos/protocol/pubsub/shm/ipc_factory.py at _open_shm_with_retry and _create_or_open, then inspect the direct SharedMemory(create=True, ...) sites around line 174. Verify that attachers wait for a nonzero segment size with a 10-second cap and produce a clear error if readiness never appears, without blind retries or a sleep ladder.