feat(sdk): gRPC streaming download fan-out + Swift core gRPC client

[Nic-dorman]

Stacked on #196 (daemon wiring + REST SDK fan-out). Review/merge that first; this branch retargets to main automatically once it lands.

What

The gRPC counterpart to the REST streaming fan-out in #196 — adds data_stream / data_stream_public over gRPC to every SDK that ships a gRPC client:

• gRPC streaming fan-out: go, py (sync + async), rust, csharp, java, kotlin, dart, ruby, cpp, elixir, swift. Proto regenerated with the new Stream RPC; idiomatic streaming return type per language. antd-js is REST-only (no gRPC client), so N/A.
• Swift core gRPC client: replaces the 15 notImplemented() stubs (health / data / chunk / file get-put-cost + the two streaming methods) with a real implementation; regenerated data.{pb,grpc}.swift; adds GrpcCoreTests.
• Ruby gRPC error-mapping fix: grpc_call constructed AntdError with positional/swapped args, so unmapped gRPC statuses raised ArgumentError instead of a mapped AntdError. Corrected, plus aligned tests and the no-gem fallback shell.

Validation

• Mock-server gRPC round-trips green on Windows across the statically- and dynamically-typed SDKs.
• dev2 (Linux): full Swift suite 57/57; Ruby 43 gRPC + 38 REST green; cpp / elixir green.
• antd-rust compiles the canonical proto (with the new Stream RPC, supplied by the feat(antd): data streaming download — daemon wiring + REST SDK fan-out #196 base) via build.rs and passes cargo fmt --check.

Notes

• On statically-typed SDKs the streaming methods stay off the shared transport interface, matching the REST fan-out's choice in feat(antd): data streaming download — daemon wiring + REST SDK fan-out #196.

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