feat: fai_dart_sdk scaffold

Dart package skeleton for the F∆I gRPC client. Used by F∆I
Studio (Tier-2) and Tier-3 domain apps (DigiScout / J∆I /
Scout). Public surface — HubClient, HubEndpoint — is committed
as a typed stub so downstream apps can compile against it
immediately. Generated proto bindings land in a follow-up
commit once the codegen step (planned: tools/generate.sh
wrapping protoc-gen-dart against ../fai_platform/proto) is
wired in.

Future Forgejo path: fai/dart-sdk.

dart analyze: clean. dart test: 4/4.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/src/hub_client.dart

@@ -0,0 +1,43 @@
+// HubClient — typed wrapper around the F∆I Hub gRPC service.
+//
+// Status (2026-05-05): scaffold. The generated proto bindings
+// from `fai/platform`'s proto/fai/v1/ directory will populate
+// the actual RPC implementations under `lib/src/generated/`
+// once the codegen tooling is wired (planned: a small
+// `tools/generate.sh` script that runs `protoc` with
+// `protoc-gen-dart` against the platform repo's protos).
+
+/// Connection coordinates for a hub endpoint.
+class HubEndpoint {
+  /// Hostname or IP. Defaults to localhost for the canonical
+  /// `fai serve` setup.
+  final String host;
+
+  /// gRPC port. Defaults to 50051.
+  final int port;
+
+  /// When true, use TLS. Phase 1+ will add credential support.
+  final bool secure;
+
+  const HubEndpoint({
+    this.host = '127.0.0.1',
+    this.port = 50051,
+    this.secure = false,
+  });
+
+  @override
+  String toString() => '${secure ? "https" : "http"}://$host:$port';
+}
+
+/// Stub client interface. The real implementation will arrive
+/// when proto codegen is wired in. Studio uses this type as the
+/// future-stable surface; the mock data layer in
+/// `fai_studio/lib/data/mock.dart` is the placeholder until then.
+class HubClient {
+  final HubEndpoint endpoint;
+
+  HubClient({this.endpoint = const HubEndpoint()});
+
+  /// Closes the underlying channel. No-op while stubbed.
+  Future<void> close() async {}
+}