feat(ipc-runtime): extract bb::ipc, add transport helpers + codegen serve() wrapper

[charlielye]

Summary

Lifts barretenberg's bb::ipc transport runtime out into a new top-level package /ipc-runtime/, peer to /ipc-codegen/. Three logical chunks landed as separate commits in this PR.

Stacks between #23314 (ipc-codegen package) and #23316 (bb.js + wsdb consumer migration). #23316 needs rebasing on top.

A. Extract bb::ipc → /ipc-runtime/cpp/

3,449 LOC moved verbatim from barretenberg/cpp/src/barretenberg/ipc/ (incl. shm/ subdir, shm.test.cpp, README) to /ipc-runtime/cpp/ipc_runtime/. Audit confirmed the module had one non-trivial external coupling — a stale throw_or_abort.hpp include in shm_server.hpp that was never actually called — which is dropped here.

The lib is consumed by barretenberg's CMake via add_subdirectory(${CMAKE_SOURCE_DIR}/../../ipc-runtime/cpp ...). 6 in-tree consumers retargeted (path rewrite + CMake link name ipc → ipc_runtime):

• nodejs_module/msgpack_client/
• api/api_msgpack.{hpp,cpp} + CMake
• bb/CMakeLists.txt (bb + bb-avm link lines)
• wsdb/{CMakeLists.txt, wsdb_ipc_server.cpp} + wsdb_client/CMakeLists.txt
• benchmark/ipc_bench/

Namespace rename bb::ipc:: → ipc:: throughout — the runtime no longer lives under barretenberg, so the bb:: prefix was historical baggage.

shm.test.cpp runs as ipc_runtime_tests under barretenberg's CTest, all 2 tests pass.

B. Transport helpers

• ipc::make_server(input_path, opts) — picks UDS vs MPSC-SHM by path suffix (.sock / .shm), matching today's hand-rolled logic in wsdb_ipc_server.cpp. Defaults match aztec-wsdb's production config (2 SHM clients, 4 MiB rings).
• ipc::install_default_signal_handlers(server) — SIGTERM/SIGINT → graceful shutdown, SIGBUS/SIGSEGV → close+exit, prctl PR_SET_PDEATHSIG on Linux, kqueue NOTE_EXIT watcher on macOS. Lifted from wsdb_ipc_server.cpp lifecycle code.

Both pure additions; no current consumer changes.

C. Codegen serve() wrapper

New CLI flag --cpp-runtime-include <prefix> (e.g. ipc_runtime) tells the C++ generator to emit a serve() that uses ipc-runtime:

template <typename Ctx>
void serve(const std::string& input_path, Ctx& ctx) {
    auto server = ipc::make_server(input_path);
    ipc::install_default_signal_handlers(*server);
    server->listen();
    auto handler = make_<service>_handler(ctx);
    server->run([&handler](int, std::span<const uint8_t> raw) {
        return handler(std::vector<uint8_t>(raw.begin(), raw.end()));
    });
}

When the flag is unset, the lightweight header-only ipc_server.hpp template path is unchanged — verified by the 18-test cross-language echo matrix still passing.

Also rename make_<service>_handler's return type from ::ipc::Handler to a service-local DispatchHandler typedef; the legacy ::ipc::Handler only existed in the lightweight template, the runtime defines a differently-shaped IpcServer::Handler, so a stable service-namespace name decouples the two backends.

Out of scope (future work)

• Truly standalone-buildable /ipc-runtime/cpp/ (its own CMakeLists without barretenberg_module_with_sources, FetchContent gtest, standalone bootstrap.sh). Today the build still relies on barretenberg's CMake tree for convenience.
• refactor(wsdb): use codegen-emitted serve() — collapses wsdb_ipc_server.cpp to ~30 LOC. Belongs after Stack B PR_B6 (wire/domain split).
• C-ABI shim + Rust/Zig wrappers (/ipc-runtime/{rust,zig}/). Defer until a second-language server consumer materialises.

Test plan

• cd ipc-codegen && ./bootstrap.sh test — 18/18 echo matrix green (default header-only path)
• Smoke-compile a generated echo server with --cpp-runtime-include ipc_runtime against ipc-runtime — compiles
• ninja ipc_runtime ipc_runtime_tests + ./bin/ipc_runtime_tests — 2/2 passing
• ninja bb bb-avm aztec-wsdb — all build with retargeted link names
• CI: full build green
• Rebase refactor(codegen): migrate bb.js BB API codegen to ipc-codegen #23316 on this branch; CI on rebased PR2 green