Sidecar: add gRPC as a third transport alongside stdio and HTTP

[thomas-stegemann]

Context

Sidecars speak JSON-RPC 2.0 over two transports today: stdio (NDJSON) and
http (POST + SSE). That choice is recorded in
ADR-0001
and stands — the entry barrier for plugin authors is the reason it was made,
and stdio remains the right default.

Two things argue for widening the choice, recorded in
ADR-0002:

• gRPC is now the default RPC stack in most sidecar languages — first-class
  in Go, mature in Rust (tonic) and Python (grpcio), and @grpc/grpc-js has
  been pure JavaScript with no native build for years.
• The streaming gap was never closed. Bowire's protocols stream heavily, but
  JSON-RPC has no stream concept, so the host rebuilds one from notifications
  ($/stream/data, $/channel/data) plus SidecarSubscriptionHub — with no
  flow control and no backpressure. ADR-0001's own "use a native .NET plugin
  above ~10k msgs/sec" is that gap stated as a limit.

Scope

Add gRPC as a third ISidecarTransport, selected by "transport": "grpc"
in sidecar.json. JSON-RPC over stdio stays the default — this is
additive, nothing existing changes behaviour.

The host side is cheap: ISidecarTransport already has two implementations
(SidecarJsonRpcTransport, SidecarHttpTransport) and the manifest already
carries a transport field. The cost is in the SDKs.

Blocked by #418

The four language SDKs are already behind the current contract. Adding a
second transport family to SDKs that have not caught up with the first
multiplies a debt that is already due. #418 lands first — this issue should
not start before it closes.

Tasks

• Proto definition for the sidecar contract (mirrors the JSON-RPC method surface)
• SidecarGrpcTransport : ISidecarTransport in the host
• "grpc" accepted in SidecarPluginManifest, discovery guard updated (cf. Sidecar discovery drops HTTP-transport plugins (guard requires executable) #415, which fixed the same class of bug for http)
• Streaming mapped onto native bidirectional streams rather than notification emulation — this is the whole point of the exercise
• SDK support per language, opt-in entry point (run_grpc() alongside run() / run_http()), so a plugin's BowirePlugin implementation is untouched
• Document in docs/architecture/sidecar-plugins.md when it ships, not before; flip ADR-0002 to Accepted
• Guidance on choosing a transport: stdio by default, http for hosted, grpc for streaming-heavy

Non-goals

• Replacing JSON-RPC. Rejected in ADR-0002: it breaks every existing sidecar and
  discards the property that a sidecar can be written in anything emitting JSON
  lines — a shell script, an exotic language, a prototype.
• Making gRPC the default.