feat: kv-store over IPC; aztec-kvdb binary; LMDBStore NAPI scaffold [PR 4]

[charlielye]

Summary

Stacked on PR 3b (#23196). Adds an out-of-process `aztec-kvdb` binary that owns LMDB and serves it over UDS / MPSC SHM with the same `TypedMessage

` wire format `yarn-project/native/MsgpackChannel` already speaks. AztecLMDBStoreV2 will be migrated from `NativeLMDBStore` (NAPI) to `KvdbBackend` (IPC) in a follow-up commit on this same PR.

This first commit is inert: the binary builds and is shipped, but nothing in yarn-project uses it yet.

Why

After PR 3b lands, only one load-bearing NAPI consumer remains: the LMDB store used by archiver, p2p, pxe, slasher, and validator-ha-signer. Moving it out of NAPI removes the last embedding of a C++ subsystem in the Node.js process. The NAPI module is reduced to a thin SHM transport stub (`MsgpackClient`/`MsgpackClientAsync`, ~400 LOC) — pure IPC plumbing, no domain logic.

SHM is the production transport (~1–10 µs round-trip via futex doorbell); UDS is the dev/test fallback. This also activates the SHM code-path that has been dead in production since PR 1.

What's in this PR (so far)

C++:

• `barretenberg/cpp/src/barretenberg/kvdb/` — `aztec-kvdb` binary. Wraps `lmdblib::LMDBStore`, runs a `bb::messaging::MessageDispatcher`, exposes the same op set as the NAPI LMDBStoreWrapper (open_database, get, has, start_cursor, advance_cursor, advance_cursor_count, close_cursor, batch, stats, copy_store).
• `barretenberg/cpp/src/barretenberg/kvdb/kvdb_messages.hpp` — wire schema. Moved from `nodejs_module/lmdb_store/lmdb_store_message.hpp`; namespace renamed `bb::nodejs::lmdb_store` → `bb::kvdb`, enum `LMDBStoreMessageType` → `KvdbMessageType`. The NAPI wrapper temporarily includes the new location until it's deleted.

TypeScript (bb.js):

• `barretenberg/ts/src/aztec-kvdb/index.ts` — `KvdbBackend`: spawns the binary, talks UDS or SHM, implements `IMsgpackBackendAsync`. Mirrors `WsdbBackend`.
• `findKvdbBinary` in `bb_backends/node/platform.ts`.
• `copy_native.sh` ships `aztec-kvdb` alongside the other binaries.

What's still ahead in this PR

• Swap `new MsgpackChannel(new NativeLMDBStore(...))` → `new MsgpackChannel(new KvdbBackend(...))` inside `yarn-project/kv-store/src/lmdb-v2/store.ts`.
• Update spawn sites (archiver, p2p, pxe, slasher, validator-ha-signer factories + `aztec-node/server.ts`) to construct/accept a `KvdbBackend`.
• Default `useShm: true` in production callers; verify the SHM transport end-to-end.
• Delete `barretenberg/cpp/src/barretenberg/nodejs_module/lmdb_store/`, the `LMDBStore` export in `init_module.cpp`, the `NativeLMDBStore` export in `@aztec/native`, and the thin `yarn-project/kv-store/src/lmdb-v2/native/` wrapper.

Stack

• PR 1 + 2a (feat(ipc): MPSC shared-memory transport with multi-client echo test #23040): MPSC SHM transport + aztec-wsdb binary — merged
• PR 2b (refactor: cut TS world state and NAPI AVM over to WSDB IPC; delete NAPI WSDB #23036): WSDB cutover
• PR 3a (feat: add generated aztec-vm-sim package setup #23084): aztec-avm + aztec-cdb binaries (inert)
• PR 3b (feat: AVM cutover — delete NAPI AVM, wire IPC simulator pool + CDB IPC server [PR 3b] #23196): AVM cutover
• PR 4 (this): kv-store IPC; LMDBStore NAPI deleted

Test plan

• `aztec-kvdb msgpack run --input /tmp/foo.sock --data-dir /tmp/lmdb-test` accepts UDS connections
• `@aztec/kv-store` `lmdb-v2` tests pass against KvdbBackend over UDS and SHM
• Bench hot `get`/cursor-page latency vs NAPI baseline (target ~5× over SHM)
• e2e `e2e_block_building` / `e2e_archiver_` / `e2e_p2p/`
• Sandbox boots and serves transactions

---

Recreated from original PR #23238. Same realignment caveats as PR above.