feat(runtime): add weft:app/notify WIT package and notify-ready host interface

Adds crates/weft-runtime/wit/weft-app.wit defining package weft:app@0.1.0
with interface notify { ready: func() }.

In the wasmtime-runtime path:
- Registers weft:app/notify@0.1.0 in the component linker before instantiation
- ready() prints

crates/weft-runtime/src/main.rs

@@ -127,6 +127,14 @@ fn run_module(
 
     let mut linker: Linker<State> = Linker::new(&engine);
     add_to_linker_sync(&mut linker).context("add WASI to linker")?;
+    linker
+        .instance("weft:app/notify@0.1.0")
+        .context("define weft:app/notify instance")?
+        .func_wrap("ready", |_: wasmtime::StoreContextMut<'_, State>, ()| {
+            println!("READY");
+            Ok::<(), wasmtime::Error>(())
+        })
+        .context("define weft:app/notify#ready")?;
 
     let mut ctx_builder = WasiCtxBuilder::new();
     ctx_builder.inherit_stdout().inherit_stderr();
@@ -150,8 +158,6 @@ fn run_module(
         },
     );
 
-    println!("READY");
-
     let command =
         Command::instantiate(&mut store, &component, &linker).context("instantiate component")?;
 

crates/weft-runtime/wit/weft-app.wit

@@ -0,0 +1,10 @@
+package weft:app@0.1.0;
+
+/// Host interface that a WEFT app component imports to signal lifecycle events.
+interface notify {
+    /// Signal to the runtime that the application has finished initialising
+    /// and is ready to serve requests.  The runtime forwards this signal to
+    /// the session supervisor (appd), which transitions the session to the
+    /// Running state.
+    ready: func();
+}