Refactor: fold DistChipProcess/DistSubWorker into WorkerThread PROCESS mode

[ChaoWao]

Summary

Absorbs the parent-side PROCESS-mode dispatch protocol (write mailbox → TASK_READY → poll TASK_DONE → IDLE) into WorkerThread, eliminating the standalone DistChipProcess and DistSubWorker IWorker subclasses. Net −208 lines.

Second half of the old PR-D (PR-D-1 landed Strict-4 ready queues in #572; this is PR-D-2). Follow-up: consolidate PROCESS-mode polling into a single multiplexed thread (currently 1:1 thread-per-child, same as before the refactor).

What changed

• WorkerThread gains Mode::THREAD | PROCESS. New dispatch_process() encodes (callable, config, args_blob) into a unified shm mailbox layout (byte-compatible with the old chip layout), signals TASK_READY, spin-polls TASK_DONE. shutdown_child() writes SHUTDOWN. Memory-ordering helpers (aarch64 ldar/stlr, x86 compiler fence, fallback __atomic) moved in from the deleted classes.
• DistWorkerManager stores WorkerEntry {worker, mode, mailbox} instead of bare IWorker *. New add_next_level_process(mailbox) / add_sub_process(mailbox) / shutdown_children().
• DistWorker gains add_process_worker(type, mailbox).
• Nanobind — DistChipProcess / DistSubWorker bindings deleted. Replaced by add_next_level_process(mailbox_ptr) / add_sub_process(mailbox_ptr) on DistWorker. DIST_MAILBOX_SIZE exported (replaces old DIST_CHIP_MAILBOX_SIZE + DIST_SUB_MAILBOX_SIZE).
• Python worker.py — unified mailbox offsets (_OFF_STATE / _OFF_CALLABLE / …); _sub_worker_loop reads callable as uint64 at offset 8; _start_level3 calls dw.add_next_level_process(addr) / dw.add_sub_process(addr); close() writes SHUTDOWN directly.
• Deleted — dist_chip_process.{h,cpp}, dist_sub_worker.{h,cpp}; CMakeLists updated.

Test plan

• pip install --no-build-isolation -e . builds cleanly.
• ctest --test-dir tests/ut/cpp/build -LE requires_hardware — 7 targets pass.
• pytest tests/ut/py/test_dist_worker/ — 21 pass.
• CI sim pipeline — run by reviewers / CI.
• Hardware L3 scene tests — run by reviewers / CI.

[gemini-code-assist]

Code Review

This pull request refactors the distributed worker system by unifying the shared-memory mailbox layout for both chip and sub-workers. It removes the explicit DistChipProcess and DistSubWorker C++ classes and their Python bindings. Instead, the WorkerThread in DistWorkerManager now handles both "THREAD mode" (direct IWorker calls) and "PROCESS mode" (IPC via the unified mailbox to pre-forked Python child processes). Python-side code (worker.py) is updated to reflect these changes, using the unified mailbox size and new add_process_worker methods. Review feedback indicates that the WorkerThread::loop in dist_worker_manager.cpp needs exception handling to prevent crashes, and a spin-poll loop in the same file could cause deadlocks if child processes become unresponsive, requiring a shutdown_ flag check. Additionally, if child process initialization fails in python/simpler/worker.py, the child should signal completion with an error code to prevent parent deadlocks.