feat(antd-cpp): gRPC external-signer prepare/finalize parity with REST

[Nic-dorman]

Summary

Adds 7 V2-284 methods on antd::GrpcClient mirroring the existing REST antd::Client external-signer surface:

• prepare_upload(path, visibility=nullopt) / prepare_upload_public(path)
• prepare_data_upload(data, visibility=nullopt)
• finalize_upload(upload_id, tx_hashes, store_data_map=false) (wave-batch)
• finalize_merkle_upload(upload_id, winner_pool_hash, store_data_map=false)
• prepare_chunk_upload(data) / finalize_chunk_upload(upload_id, tx_hashes)

Includes the canonical store_data_map=true variant — cpp keeps that param (unlike kotlin/swift).

Depends on #140.

Implementation notes

• New UploadService::Stub on the existing pimpl. Two free helpers (map_prepare_upload_response, map_finalize_upload_response) isolate the wire-shape mapping.
• Merkle-only fields (depth, pool_commitments, merkle_payment_timestamp) are gated on payment_type == "merkle" — proto3 scalar defaults are not enough because REST omits these entirely on wave-batch and the model layer expects them empty/zero there.
• visibility is sent only when std::optional has a value, preserving the proto3 empty-default wire shape for daemons predating the public-prepare addition.
• prepare_chunk_upload short-circuits the wave-batch mapping when already_stored == true.

Tests

New file tests/test_grpc_external_signer.cpp. Spins up a real grpc++ server on 127.0.0.1:0 with MockUploadService + MockChunkService, then dials with a real antd::GrpcClient. Exercises the actual proto wire-shape mapping (visibility round-trip via upload_id encoding, MERKLE vs wave-batch branch, EXISTS short-circuit).

12 new tests parallel to the antd-rust / antd-go / antd-py / antd-java / antd-kotlin / antd-csharp / antd-ruby / antd-dart / antd-swift suites — full canonical matrix including store_data_map=true.

Gated on -DANTD_BUILD_GRPC=ON since they link against the grpc++ service stubs. The existing isolated antd_grpc_tests stays unchanged and still runs on the default config.

Gates passed on dev2

cmake -B build -DANTD_BUILD_GRPC=ON -DProtobuf_PROTOC_EXECUTABLE=/usr/bin/protoc
cmake --build build
ctest --test-dir build

• cmake --build — clean
• ctest — 4/4 passing (antd_tests, antd_async_tests, antd_grpc_tests isolated, antd_grpc_external_signer_tests 12/12 V2-284 mock-server wire-mapping tests)
• Default config (no flags) still builds and passes 3/3 tests — the new test target is fully gated on ANTD_BUILD_GRPC

Note: on dev2, point CMake at the apt-installed protoc — there is also a newer /usr/local/bin/protoc 25.x for swift codegen, which would produce stubs incompatible with the apt-installed libprotobuf 3.21.x runtime.

Test plan

• Once feat(antd): gRPC UploadService + chunks prepare/finalize #140 lands: rebase and re-run ctest against the real daemon protos
• Verify the build on a clean install where both protoc binaries do not coexist (i.e. only one is on PATH)