feat(ipc): MPSC shared-memory transport with multi-client echo test

[charlielye]

Part 1 — MPSC shared-memory transport (originally cl/ipc-1-mpsc-shm)

Adds a multi-producer, single-consumer shared-memory IPC transport. Internally this is N SPSC rings (one per producer) + a shared doorbell futex — there is no shared producer ring. Each client writes only to its own SPSC ring; producers ring the doorbell to wake the consumer, which then checks which ring has data and reads from it. Each client also has its own SPSC response ring.

This is the same pattern that existed in earlier IPC iterations and is what aztec-wsdb will/could use when serving both the TS layer and a C++ AVM worker pool from the same world state if we need the performance.

Right now though we'll use UDS. This is just if needed in future.

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Part 2 — aztec-wsdb binary + IPC supporting code (originally #23035 / cl/ipc-2-wsdb, merged into this PR)

Summary

Adds the standalone aztec-wsdb binary plus all supporting code (C++ client library, TS spawner, IPC adapter) needed to move world state out of the Node.js process. This part is inert: nothing yet uses the new binary. A follow-up PR (cl/ipc-3-avm-wsdb-cutover, #23036) cuts the NAPI AVM and the TS world state over to use it.

What's added

C++:

• barretenberg/cpp/src/barretenberg/wsdb/ — aztec-wsdb standalone binary that runs the world state DB as an IPC server. Same WorldState surface as the in-process NAPI module, but exposed over msgpack via UDS or shared memory.
• barretenberg/cpp/src/barretenberg/wsdb_client/ — WsdbIpcMerkleDB implements LowLevelMerkleDBInterface over WSDB IPC. The standalone AVM (or NAPI AVM after cutover) will use this in place of an in-process WorldState reference.
• barretenberg/cpp/src/barretenberg/ipc/mpsc_shm_{client,server}.hpp — multi-producer single-consumer shared-memory transport (see Part 1 above). Lower latency than UDS for the AVM↔WSDB hop.

TypeScript (bb.js):

• barretenberg/ts/src/aztec-wsdb/ — WsdbBackend spawns the aztec-wsdb binary and routes msgpack commands via the generated AsyncApi. Implements IMsgpackBackendAsync.
• barretenberg/ts/src/cbind/cpp_codegen.ts — C++ codegen used by aztec-wsdb's generate.ts to produce wsdb_ipc_client_generated.{cpp,hpp}. Small shared updates to schema_visitor / typescript_codegen / rust_codegen.

yarn-project:

• yarn-project/world-state/src/native/ipc_world_state_instance.ts — IpcWorldState implements NativeWorldStateInstance over WSDB IPC. Not yet wired in.

Why split this way

The full WSDB-out-of-process cutover involves rewiring the NAPI AVM (which currently dereferences an in-process WorldState* pointer) to talk to aztec-wsdb over IPC, plus replacing TS NAPI WorldState with IpcWorldState everywhere it's used. This PR keeps the diff bounded by landing the binary and supporting code first; the cutover lands separately and should be a tiny diff.

Verification

• aztec-wsdb builds: cd barretenberg/cpp/build && ninja aztec-wsdb
• wsdb_client static library builds: ninja wsdb_client
• bb.js builds (esm/cjs/browser): cd barretenberg/ts && yarn build:esm && yarn build:cjs && yarn build:browser
• @aztec/world-state typechecks clean (the only TS errors in the build output are pre-existing on next in unrelated packages)

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Stack

This is part of a stack splitting up #21331. Plan: /mnt/user-data/charlie/.claude/plans/glittery-snuggling-horizon.md.

• PR 1 + 2a (this): MPSC SHM transport + aztec-wsdb binary + supporting code (inert)
• PR 2b (refactor: cut TS world state and NAPI AVM over to WSDB IPC; delete NAPI WSDB #23036): NAPI AVM + TS world state cutover — already merged
• PR 3a (feat: add aztec-avm + aztec-cdb binaries and IPC supporting code (inert) #23084): standalone aztec-avm + CDB IPC server (inert)
• PR 3b (feat: AVM cutover — delete NAPI AVM, wire IPC simulator pool + CDB IPC server [PR 3b] #23154): AVM cutover; delete NAPI AVM; fix partial-note bug; ship pool + cancellation atomically