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/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,