Merge v3-t17: Nix package with the complete userland, builder closure and seeded world tree; v2 removed (T17)

# Conflicts:
#	README.md

host/src/world.rs

@@ -5,6 +5,7 @@
 //! - `host/`: host state outside the sandbox (harness storage, later the vault). No guest sees it.
 use sausage_vfs::{Device, Generated, Host, Layer, Memory, Mounts, Process};
 use std::{
+    os::unix::fs::PermissionsExt,
     path::{Path, PathBuf},
     sync::Arc,
 };
@@ -21,6 +22,9 @@ pub struct World {
 
 fn copy_tree(source: &Path, destination: &Path) -> std::io::Result<()> {
     std::fs::create_dir_all(destination)?;
+    let mut permissions = std::fs::metadata(destination)?.permissions();
+    permissions.set_mode(permissions.mode() | 0o200);
+    std::fs::set_permissions(destination, permissions)?;
     for entry in std::fs::read_dir(source)? {
         let entry = entry?;
         let target = destination.join(entry.file_name());
@@ -29,7 +33,10 @@ fn copy_tree(source: &Path, destination: &Path) -> std::io::Result<()> {
         } else if entry.file_type()?.is_dir() {
             copy_tree(&entry.path(), &target)?;
         } else {
-            std::fs::copy(entry.path(), target)?;
+            std::fs::copy(entry.path(), &target)?;
+            let mut permissions = std::fs::metadata(&target)?.permissions();
+            permissions.set_mode(permissions.mode() | 0o200);
+            std::fs::set_permissions(&target, permissions)?;
         }
     }
     Ok(())
@@ -127,3 +134,53 @@ impl World {
         })
     }
 }
+
+#[cfg(test)]
+#[test]
+fn readonly_bundle_copies_are_writable() -> std::io::Result<()> {
+    use std::fs;
+    let bundle = tempfile::tempdir()?;
+    let world = tempfile::tempdir()?;
+    for (name, mode) in [("bin/tool.wasm", 0o555), ("src/lib.rs", 0o444)] {
+        let path = bundle.path().join(name);
+        fs::create_dir_all(path.parent().unwrap())?;
+        fs::write(&path, b"original")?;
+        fs::set_permissions(path, fs::Permissions::from_mode(mode))?;
+    }
+    for path in [
+        bundle.path().join("bin"),
+        bundle.path().join("src"),
+        bundle.path().to_owned(),
+        world.path().to_owned(),
+    ] {
+        fs::set_permissions(path, fs::Permissions::from_mode(0o555))?;
+    }
+    let result = copy_tree(bundle.path(), world.path());
+    // Restore source directories so the temporary directory can remove its files.
+    for path in [
+        bundle.path(),
+        &bundle.path().join("bin"),
+        &bundle.path().join("src"),
+    ] {
+        fs::set_permissions(path, fs::Permissions::from_mode(0o755))?;
+    }
+    result?;
+    assert_eq!(
+        fs::metadata(world.path())?.permissions().mode() & 0o777,
+        0o755
+    );
+    for (name, mode) in [("bin/tool.wasm", 0o555), ("src/lib.rs", 0o444)] {
+        let source = bundle.path().join(name);
+        let target = world.path().join(name);
+        assert_eq!(fs::metadata(&source)?.permissions().mode() & 0o777, mode);
+        assert_eq!(
+            fs::metadata(&target)?.permissions().mode() & 0o777,
+            mode | 0o200
+        );
+        fs::write(&target, b"changed")?;
+        assert_eq!(fs::read(source)?, b"original");
+    }
+    fs::write(world.path().join("bin/added.wasm"), b"new")?;
+    fs::write(world.path().join("src/added.rs"), b"new")?;
+    Ok(())
+}