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>

README.md

@@ -0,0 +1,52 @@
+# fai_dart_sdk
+
+gRPC client SDK for the F∆I Platform hub. Used by F∆I Studio
+(Tier-2 generic GUI) and any Tier-3 Dart/Flutter domain app
+(DigiScout, J∆I, Scout, …) that wants to talk to a running
+`fai serve`.
+
+> **Status (2026-05-05):** scaffold. Public surface (`HubClient`,
+> `HubEndpoint`) is committed; generated proto bindings land in
+> a follow-up commit once the codegen step is wired in. Until
+> then, downstream apps can compile against the stub and Studio
+> uses mock data.
+
+## Why a separate SDK package
+
+The platform hub speaks gRPC. Each Dart consumer (Studio plus
+each Tier-3 domain app) would otherwise re-generate proto
+bindings independently and write near-identical client
+boilerplate. This package centralises that work:
+
+- One source of generated proto bindings.
+- One typed `HubClient` wrapper with helpers.
+- One place to bump when the wire protocol evolves.
+
+## Layout
+
+```
+fai_dart_sdk/
+├── lib/
+│   ├── fai_dart_sdk.dart          # Public API
+│   └── src/
+│       ├── hub_client.dart        # Typed client wrapper
+│       └── generated/             # protoc output (committed; pinned to platform tag)
+├── tools/
+│   └── generate.sh                # Wraps `protoc --dart_out=...` against ../fai_platform/proto
+├── example/
+└── pubspec.yaml
+```
+
+## Repo placement
+
+Will be published as `fai/dart-sdk` on Forgejo
+(`git.flemming.ws`). The local directory `fai_dart_sdk/`
+follows the established `fai_platform/` layout convention.
+
+## Versioning
+
+Tracks the platform's wire-protocol version, not the platform
+binary version. As long as `fai:platform@1.x` is current, this
+SDK stays at `0.x` (will move to `1.0` once API stabilises);
+when `fai:platform@2.0` lands, a `2.x` SDK ships in parallel
+during the transition.