refactor(bbapi): adopt codegen-emitted dispatch; wire-typed handler boundary

[charlielye]

Summary

Stacked on #23522 → #23499 → #23498 → #23497 → #23316.

api_msgpack.cpp's hand-rolled msgpack dispatch lambda (~80 LOC of length-prefix decode + NamedUnion convert + bbapi() call + error wrapping) is replaced by `make_bb_handler(request)` from the codegen-emitted bb_ipc_server.hpp. Same wire format, same in-process semantics — BBApiRequest is kept at server-function scope so IVC state is preserved across the call sequence (ChonkStart → ChonkLoad → ChonkAccumulate → ChonkProve).

Codegen wiring

Added C++ codegen step in `bbapi/CMakeLists.txt` (analogous to wsdb's). Emits `generated/bb_{types,ipc_client,ipc_server}.hpp` + `bb_ipc_client.cpp` from `bb_schema.json`. `bbapi_objects` gets ipc_runtime as a PUBLIC link dep since the generated headers include `ipc_runtime/ipc_server.hpp`.

Handler implementation

The codegen-emitted `make_bb_handler` template instantiates a dispatch table that calls `handle_(BBApiRequest&, wire::Cmd&&)` overloads. Two new files provide them:

• `bbapi_handlers.hpp` — 60 non-template overload declarations (one per non-Shutdown command in bb_schema.json).
• `bbapi_handlers.cpp` — 60 one-liner definitions:

  wire::FooResponse handle_foo(BBApiRequest& ctx, wire::Foo&& cmd) {
      return roundtrip<wire::FooResponse>(roundtrip<Foo>(cmd).execute(ctx));
  }

The wire types (from codegen) and domain types (`bbapi_*.hpp`) share a SERIALIZATION_FIELDS shape — same field names, msgpack-compatible field types (`Fr` packs as bin32 matching `bb::fr`; `GrumpkinPoint`/`Bn254G1Point`/etc. pack as {x, y} maps matching `affine_element`). So pack-then-unpack converts between them with no information loss. Slower than field-by-field copying (one extra pack+unpack pair per direction) but trivial to write and resistant to schema drift.

Test plan

• `ninja bb bb-avm aztec-wsdb ipc_runtime_tests bbapi_tests api_tests` — clean
• `./bin/bbapi_tests` — 30/30 tests pass (msgpack default-roundtrip suite covering CircuitProve, ChonkProve, all structural commands + CBind exception handling)
• `./bin/ipc_runtime_tests` — 2 SHM tests pass
• ipc-codegen cross-language matrix — 18/18 green
• CI: bb-prover e2e + ivc-integration (live wire-format check)

What's deferred to PR-F

The hand-written types in `bbapi_{crypto,ecc,ecdsa,schnorr,chonk,ultra_honk,avm,srs}.hpp` still exist alongside the codegen-emitted wire types. PR-F will:

• Replace each handler's `msgpack_roundtrip` with field-by-field wire→domain conversion
• Delete the hand-written type definitions (keep utility funcs in `bbapi_shared.hpp` and the execute-method bodies, re-shaped as free functions)
• Delete `bbapi_execute.{hpp,cpp}` (NamedUnion + visitor)
• Update `c_bind.cpp` (the WASM CBIND entry point) and `ipc.bench.cpp` to use the codegen dispatch

That's a mechanical follow-up; this PR establishes the wire-typed handler boundary as the contract.