Add launcher engine (scopenet-core) and admin panel server

- scopenet-shared: wire types for branding, auth, instances, manifests
- scopenet-core: Mojang versions/libraries/assets, automatic Java runtimes,
  Fabric/Quilt/Forge/NeoForge installation, file sync, launch command
  builder, Microsoft auth, servers.dat injection, server list ping
- scopenet-panel: Axum + SQLite admin API with JWT auth, users/groups,
  branding, settings, instances, Modrinth/CurseForge/zip import, file
  hosting and launch stats

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ARcGWxLx21FwXJ3yfGriS

crates/core/src/paths.rs

@@ -0,0 +1,99 @@
+use std::path::{Path, PathBuf};
+
+/// On-disk layout. Versions, libraries, assets and Java runtimes are shared
+/// between instances so a second instance of the same version costs almost
+/// no disk space.
+#[derive(Debug, Clone)]
+pub struct Layout {
+    pub root: PathBuf,
+}
+
+impl Layout {
+    pub fn new(root: impl Into<PathBuf>) -> Self {
+        Self { root: root.into() }
+    }
+    pub fn libraries(&self) -> PathBuf {
+        self.root.join("libraries")
+    }
+    pub fn versions(&self) -> PathBuf {
+        self.root.join("versions")
+    }
+    pub fn version_dir(&self, id: &str) -> PathBuf {
+        self.versions().join(id)
+    }
+    pub fn version_json(&self, id: &str) -> PathBuf {
+        self.version_dir(id).join(format!("{id}.json"))
+    }
+    pub fn version_jar(&self, id: &str) -> PathBuf {
+        self.version_dir(id).join(format!("{id}.jar"))
+    }
+    pub fn assets(&self) -> PathBuf {
+        self.root.join("assets")
+    }
+    pub fn runtimes(&self) -> PathBuf {
+        self.root.join("runtimes")
+    }
+    pub fn natives(&self, version_id: &str) -> PathBuf {
+        self.root.join("natives").join(version_id)
+    }
+    pub fn instances(&self) -> PathBuf {
+        self.root.join("instances")
+    }
+    pub fn instance_dir(&self, id: &str) -> PathBuf {
+        self.instances().join(sanitize_id(id))
+    }
+    pub fn cache(&self) -> PathBuf {
+        self.root.join("cache")
+    }
+}
+
+/// Keep instance ids filesystem-safe no matter what the panel sends.
+pub fn sanitize_id(id: &str) -> String {
+    let s: String = id
+        .chars()
+        .map(|c| if c.is_ascii_alphanumeric() || c == '-' || c == '_' || c == '.' { c } else { '_' })
+        .collect();
+    let s = s.trim_matches('.').to_string();
+    if s.is_empty() { "_".into() } else { s }
+}
+
+/// Join a server-provided relative path onto `base`, refusing anything that
+/// could escape it (`..`, absolute paths, drive letters).
+pub fn safe_join(base: &Path, rel: &str) -> Option<PathBuf> {
+    let rel = rel.replace('\\', "/");
+    if rel.starts_with('/') || rel.contains(':') {
+        return None;
+    }
+    let mut out = base.to_path_buf();
+    for part in rel.split('/') {
+        match part {
+            "" | "." => continue,
+            ".." => return None,
+            p => out.push(p),
+        }
+    }
+    if out == base { None } else { Some(out) }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn safe_join_blocks_traversal() {
+        let base = Path::new("/game");
+        assert_eq!(safe_join(base, "mods/a.jar"), Some(PathBuf::from("/game/mods/a.jar")));
+        assert_eq!(safe_join(base, "config\\x.toml"), Some(PathBuf::from("/game/config/x.toml")));
+        assert!(safe_join(base, "../evil").is_none());
+        assert!(safe_join(base, "mods/../../evil").is_none());
+        assert!(safe_join(base, "/etc/passwd").is_none());
+        assert!(safe_join(base, "C:/Windows").is_none());
+        assert!(safe_join(base, "").is_none());
+    }
+
+    #[test]
+    fn ids_are_sanitized() {
+        assert_eq!(sanitize_id("my pack/../x"), "my_pack_.._x");
+        assert_eq!(sanitize_id(".."), "_");
+    }
+}