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

[Nic-dorman]

Summary

Adds 7 V2-284 functions on Antd.GrpcClient mirroring the existing REST Antd.Client external-signer surface:

• prepare_upload(client, path, opts) / prepare_upload_public(client, path)
• prepare_data_upload(client, data, opts)
• finalize_upload(client, upload_id, tx_hashes) (wave-batch)
• finalize_merkle_upload(client, upload_id, winner_pool_hash, opts)
• prepare_chunk_upload(client, data) / finalize_chunk_upload(client, upload_id, tx_hashes)

Bang variants throughout. Full canonical 12-test matrix including store_data_map: true.

Depends on #140.

Proto regen

• New lib/antd/v1/upload.pb.ex — UploadService + request/response messages + PoolCommitmentEntry + CandidateNodeEntry.
• lib/antd/v1/chunks.pb.ex extended with PrepareChunk + FinalizeChunk RPCs and request/response messages.
• lib/antd/v1/common.pb.ex extended with PaymentEntry.
• All .pb.ex regenerated with protoc-gen-elixir 0.16.0 (was 0.13.0). The bump fixes a latent runtime bug: protobuf 0.16 dropped the generated .new/1 constructor. All calls inside Antd.GrpcClient are now %Module{...} struct literals (the forward-compatible idiom). The existing gRPC functions did not exercise this at runtime because the test suite simulates rather than calls the protobuf modules.

Implementation notes

• 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 fields entirely on wave-batch.
• visibility is forwarded as the proto3 default "" when not supplied via opts.
• prepare_chunk_upload short-circuits the wave-batch mapping when already_stored == true.

Tests

New file test/antd/grpc_external_signer_test.exs. Spins up a real grpc-elixir server on 127.0.0.1:0 via GRPC.Server.start_endpoint(TestEndpoint, 0) with MockUploadServer + MockChunkServer, 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 / antd-cpp suites.

The suite starts GRPC.Client.Supervisor in setup_all — grpc-elixir 0.11 does not start it as part of its application supervision tree, and Antd.GrpcClient.new/1 calls GRPC.Stub.connect/1 which requires it. async: false because the supervisor is process-global.

Gates passed on dev2

• mix compile — clean
• mix test — 86/86 passing (74 existing + 12 new V2-284)

Test plan

• Once feat(antd): gRPC UploadService + chunks prepare/finalize #140 lands: rebase and re-run mix test against the real daemon protos
• Confirm the protoc_gen_elixir_version bump (0.13 → 0.16) does not break downstream applications that depend on antd and construct request types directly (none expected — generated *.pb.ex modules are not part of the public surface)