feat(credentials): manage declarative credentials through TOML (#2515)

* feat(credentials): manage declarative credentials through TOML

Make managed credentials part of the canonical TOML configuration and
load them before peers can authenticate.

Reuse ConfigRpc hot patches to durably replace the configured credential
set without restarting the instance. Serialize credential mutations so
base, managed, and ephemeral keys cannot race into conflicts.

Remove the managed overlay file format, digest protocol, capability
negotiation, force reconciliation, and database CAS machinery. Redact
credential secrets from debug output and management events. Write
credential-bearing files atomically with private permissions.

* fix(core): release JoinSet reapers with their owners

Pass weak task-set references into background reapers so they cannot
retain the JoinSet they are meant to collect. This lets stale smoltcp
bridge tasks terminate when an IPv4 generation is replaced.

Add ownership and TCP generation-replacement regressions covering the
production port-forward failure.

easytier/src/common/credential_manager.rs

@@ -1,6 +1,6 @@
 use std::{io::Write, path::PathBuf, sync::Arc};
 
-use atomic_write_file::AtomicWriteFile;
+use atomic_write_file::{AtomicWriteFile, OpenOptions};
 use easytier_core::peers::credential_manager::CredentialStorage;
 
 struct FileCredentialStorage {
@@ -9,21 +9,33 @@ struct FileCredentialStorage {
 
 impl CredentialStorage for FileCredentialStorage {
     fn load(&self) -> anyhow::Result<Option<String>> {
-        let Ok(serialized) = std::fs::read_to_string(&self.path) else {
-            return Ok(None);
+        let serialized = match std::fs::read_to_string(&self.path) {
+            Ok(serialized) => serialized,
+            Err(error) if error.kind() == std::io::ErrorKind::NotFound => return Ok(None),
+            Err(error) => return Err(error.into()),
         };
         tracing::info!(path = %self.path.display(), "loaded credentials");
         Ok(Some(serialized))
     }
 
     fn store(&self, serialized_credentials: &str) -> anyhow::Result<()> {
-        let mut file = AtomicWriteFile::open(&self.path)?;
+        let mut file = restricted_atomic_file(&self.path)?;
         file.write_all(serialized_credentials.as_bytes())?;
         file.commit()?;
         Ok(())
     }
 }
 
+fn restricted_atomic_file(path: &std::path::Path) -> std::io::Result<AtomicWriteFile> {
+    let mut options = OpenOptions::new();
+    #[cfg(unix)]
+    {
+        atomic_write_file::unix::OpenOptionsExt::preserve_mode(&mut options, false);
+        std::os::unix::fs::OpenOptionsExt::mode(&mut options, 0o600);
+    }
+    options.open(path)
+}
+
 pub(crate) fn runtime_credential_storage(
     path: Option<PathBuf>,
 ) -> Option<Arc<dyn CredentialStorage>> {
@@ -48,5 +60,28 @@ mod tests {
             storage.load().unwrap().as_deref(),
             Some("{\"credential\":false}")
         );
+        #[cfg(unix)]
+        {
+            use std::os::unix::fs::PermissionsExt as _;
+
+            assert_eq!(
+                std::fs::metadata(&storage.path)
+                    .unwrap()
+                    .permissions()
+                    .mode()
+                    & 0o777,
+                0o600
+            );
+        }
+    }
+
+    #[test]
+    fn file_storage_reports_read_errors() {
+        let directory = tempfile::tempdir().unwrap();
+        let storage = FileCredentialStorage {
+            path: directory.path().to_path_buf(),
+        };
+
+        assert!(storage.load().is_err());
     }
 }