feat: v0.1.0 — Studio Plugin SDK

First release of the SDK that Studio plugins depend on.

What v0.1 ships:

- WIT contract under wit/ (studio-plugin.wit +
  deps/platform/world.wit, both verbatim from
  fai/platform/wit/).
- wit-bindgen 0.36 generate! call with pub_export_macro
  enabled so the emitted __export_studio_plugin_impl! macro
  is reachable from downstream plugin crates.
- export_plugin!(MyType) convenience macro that wraps the
  bindgen-generated impl macro.
- README documenting usage + v0.1 limits (single-world ->
  all-three-hooks-required, no proc-macro yet, edition
  2021 pinned by wit-bindgen output).

What v0.1 doesn't yet do:

- Plugins still declare all three Guest impls (theme,
  translate, output-view). v0.2 splits the WIT into one
  world per hook.
- No #[plugin(...)] proc-macro. v0.2 wraps generate! +
  Guest impl generation into a single attribute.
- Host-side: the hub doesn't yet load studio-plugin-world
  components or expose InvokePlugin RPC — that's the next
  push.

Validated against the in-tree studio-theme-solarized
plugin: 22 KiB stripped .wasm artefact, valid Component
Model, passes wasm-tools component wit inspection.

Signed-off-by: flemming-it <sf@flemming.it>

README.md

@@ -0,0 +1,107 @@
+# fai-studio-plugin-sdk
+
+Rust SDK for writing F∆I Studio plugins.
+
+## Status
+
+**v0.1 builds.** Studio plugins targeting the `studio-plugin`
+world declared in `wit/studio-plugin.wit` can be compiled to
+`wasm32-wasip2` against this crate today. The proof-of-
+contract plugin is `studio-theme-solarized` in the
+fai-modules namespace.
+
+What's NOT yet implemented:
+
+- **Hub side**: a Component-Model loader path for
+  `studio-plugin`-world components + the `InvokePlugin`
+  gRPC RPC that Studio talks to. Tracked in
+  `fai/platform/docs/advanced/studio-plugin-sdk.md`.
+- **Studio side**: `plugin_host.dart` that discovers
+  installed `studio.*` capabilities and dispatches.
+- **Per-hook worlds**: today's v0.1 ships a single world
+  (`studio-plugin`) that exports all three hooks
+  (`theme`, `translate`, `output-view`). A plugin only
+  caring about one hook still has to declare empty /
+  declined impls for the other two. v0.2 splits into one
+  world per hook.
+- **`#[plugin(...)]` proc-macro**: v0.1 ships a plain
+  `export_plugin!` macro that wraps the wit-bindgen-emitted
+  `__export_studio_plugin_impl!`. The proc-macro that
+  generates the WIT bindings + the `Guest` impls in one
+  pass arrives in v0.2.
+
+## Usage
+
+```toml
+# Cargo.toml of your plugin
+[package]
+name = "my_studio_plugin"
+version = "0.1.0"
+edition = "2021"   # match SDK edition
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+fai-studio-plugin-sdk = { git = "https://git.flemming.ai/fai/studio-plugin-sdk", branch = "main" }
+```
+
+```rust
+// src/lib.rs of your plugin
+use fai_studio_plugin_sdk::{bindings, export_plugin};
+
+type ColorScheme = bindings::fai::studio_plugin::plugin_types::ColorScheme;
+type PluginError = bindings::fai::studio_plugin::plugin_types::PluginError;
+type RenderedOutput = bindings::fai::studio_plugin::plugin_types::RenderedOutput;
+
+struct MyPlugin;
+
+impl bindings::exports::fai::studio_plugin::theme::Guest for MyPlugin {
+    fn theme_for(brightness: String) -> Result<ColorScheme, PluginError> {
+        // Return a ColorScheme or PluginError::Declined.
+    }
+}
+
+// v0.1: every plugin must declare all three hooks. Plugins
+// that don't actually serve a hook return Declined.
+impl bindings::exports::fai::studio_plugin::translate::Guest for MyPlugin {
+    fn translate(_: String, _: String, _: String) -> Result<String, PluginError> {
+        Err(PluginError::Declined("not a translate plugin".to_string()))
+    }
+}
+impl bindings::exports::fai::studio_plugin::output_view::Guest for MyPlugin {
+    fn render(_: String, _: Vec<u8>) -> Result<RenderedOutput, PluginError> {
+        Err(PluginError::Declined("not a view plugin".to_string()))
+    }
+}
+
+export_plugin!(MyPlugin);
+```
+
+Then:
+
+```bash
+cargo build --release --target wasm32-wasip2
+fai pack . -o my-plugin-0.1.0.fai
+```
+
+## WIT layout
+
+```
+wit/
+├── studio-plugin.wit       # main world, fai:studio-plugin@0.1.0
+└── deps/
+    └── platform/
+        └── world.wit       # fai:platform@1.0.0 (host imports)
+```
+
+Both files are verbatim copies of the authoritative versions
+in `fai/platform/wit/`. When the contract changes upstream,
+re-copy here in lock-step.
+
+## Why edition 2021
+
+wit-bindgen 0.36 emits `#[link_section]` / unsafe-attribute
+syntax that the edition-2024 compiler rejects. Bump when
+moving to a wit-bindgen version with edition-2024-compatible
+codegen.