Add UMBP standalone process mode

[maning00]

Summary

Adds UMBP standalone process mode so worker processes can access a shared UMBP server over gRPC/UDS with shm fd handoff, instead of embedding all local storage state in each worker.

Key changes:

• Add standalone-process config, client factory wiring, pybind exports, and Python package exposure.
• Add umbp_standalone_server, StandaloneProcessClient, standalone protobuf service, and raw UDS fd-passing IPC.
• Add AnonymousShm host-buffer backing and registry support for zero-copy worker buffer registration.
• Add backend-mode reporting and distributed-backed server support via CreateUMBPClient.
• Add per-worker ExternalKvIdentityClient support for distributed-backed external-KV identity forwarding.
• Document the standalone-process design, distributed-backed extension, lifecycle ordering, and accepted lifecycle-lock tradeoff.
• Add standalone shm IPC coverage and CopyPipeline drain coverage.

Testing

• pre-commit run
• cmake --build build --target umbp_standalone_server test_standalone_shm_ipc mori_pybinds -j$(nproc)
• ./src/umbp/tests/test_standalone_shm_ipc --gtest_color=no
• cd build/src/umbp/tests && ctest --output-on-failure (174/174 passed)