Merge v3-t39: Rust builder behind host.sausage/cargo and the world cargo CLI (T39, P4)

# Conflicts:
#	flake.nix
#	host/src/process/mod.rs
#	host/tests/hook_git.rs
#	host/tests/hook_guest.rs
#	host/tests/process.rs
#	userland/Cargo.toml

host/src/api/cargo.rs

@@ -0,0 +1,60 @@
+//! A streaming Cargo response. Dropping the body cancels the builder's native process group.
+use crate::builder::Event;
+use bytes::Bytes;
+use http::{Request, Response};
+use http_body_util::BodyExt;
+use std::{
+    pin::Pin,
+    task::{Context, Poll},
+};
+struct Body(tokio::sync::mpsc::Receiver<Event>);
+impl http_body::Body for Body {
+    type Data = Bytes;
+    type Error = wasmtime_wasi_http::Error;
+    fn poll_frame(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Result<http_body::Frame<Bytes>, Self::Error>>> {
+        self.0.poll_recv(cx).map(|event| {
+            event.map(|event| {
+                let mut bytes = serde_json::to_vec(&event).expect("Cargo event");
+                bytes.push(b'\n');
+                Ok(http_body::Frame::data(Bytes::from(bytes)))
+            })
+        })
+    }
+}
+pub(crate) fn stream(
+    api: &super::Api,
+    request: Request<Bytes>,
+) -> Response<wasmtime_wasi_http::WasiBody> {
+    let (tx, rx) = tokio::sync::mpsc::channel(16);
+    let builder = api.builder.get().cloned();
+    let request = serde_json::from_slice::<crate::builder::Request>(request.body());
+    tokio::spawn(async move {
+        let result = async {
+            builder
+                .ok_or_else(|| anyhow::anyhow!("builder unavailable"))?
+                .execute(request?, &tx)
+                .await
+        }
+        .await;
+        if let Err(error) = result {
+            let _ = tx
+                .send(Event::Stderr {
+                    bytes: format!("cargo: {error:#}\n").into_bytes(),
+                })
+                .await;
+            let _ = tx
+                .send(Event::Exit {
+                    code: 1,
+                    executables: Vec::new(),
+                })
+                .await;
+        }
+    });
+    Response::builder()
+        .header("content-type", "application/x-ndjson")
+        .body(Body(rx).boxed_unsync())
+        .unwrap()
+}

host/src/api/mod.rs

@@ -1,6 +1,8 @@
 //! JSON routes for host.sausage. The transport supplies the trusted caller.
 //! Host-owned pages use passkeys and typed proofs for every authorization ceremony.
 mod auth;
+mod cargo;
+pub(crate) use cargo::stream;
 mod pages;
 mod server;
 mod ssh;
@@ -20,6 +22,7 @@ use std::{
 
 pub struct Api {
     pub cred: Arc<Cred>,
+    pub(crate) builder: std::sync::OnceLock<Arc<crate::builder::Builder>>,
     script: cred::quickjs::QuickJs,
     pub passkeys: Mutex<Passkeys<rusqlite::Connection>>,
     pub entry: EntryKey,
@@ -49,6 +52,7 @@ impl Api {
     ) -> Self {
         Self {
             cred: cred.into(),
+            builder: Default::default(),
             script: cred::quickjs::QuickJs::default(),
             passkeys: Mutex::new(passkeys),
             entry,