refactor(daemon): centralize username path-segment sanitizer

Extract the inline path-traversal check from PendingStore into a shared
storage::safe_user::join_user_segment helper. RecoveryStore (next) reuses it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

daemon/src/storage/mod.rs

@@ -1,4 +1,5 @@
 pub(crate) mod credentials;
 pub(crate) mod pending;
 pub(crate) mod policy;
+pub(crate) mod safe_user;
 pub(crate) mod userdb;

daemon/src/storage/pending.rs

@@ -24,15 +24,9 @@ impl PendingStore {
     }
 
     fn user_path(&self, user: &str) -> Result<PathBuf, PendingError> {
-        if user.is_empty()
-            || user.contains('/')
-            || user.contains('\0')
-            || user == "."
-            || user == ".."
-        {
-            return Err(PendingError::InvalidUser(user.to_string()));
-        }
-        Ok(self.dir.join(user))
+        super::safe_user::join_user_segment(&self.dir, user).map_err(|e| match e {
+            super::safe_user::SegmentError::Invalid(s) => PendingError::InvalidUser(s),
+        })
     }
 
     pub fn set(&self, user: &str, flag: &PendingFlag) -> Result<(), PendingError> {

daemon/src/storage/safe_user.rs

@@ -0,0 +1,45 @@
+//! Shared username sanitizer for path-segment use across pending/recovery stores.
+
+use std::path::{Path, PathBuf};
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub(crate) enum SegmentError {
+    #[error("invalid username segment: {0:?}")]
+    Invalid(String),
+}
+
+/// Reject any username that could escape a single path segment. Centralized
+/// so pending / recovery / future stores don't drift on what's "safe."
+pub(crate) fn join_user_segment(dir: &Path, user: &str) -> Result<PathBuf, SegmentError> {
+    if user.is_empty()
+        || user == "."
+        || user == ".."
+        || user.contains('/')
+        || user.contains('\0')
+    {
+        return Err(SegmentError::Invalid(user.to_string()));
+    }
+    Ok(dir.join(user))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn accepts_normal_username() {
+        let p = join_user_segment(Path::new("/tmp/x"), "alice").unwrap();
+        assert_eq!(p, PathBuf::from("/tmp/x/alice"));
+    }
+
+    #[test]
+    fn rejects_traversal_and_separators() {
+        for evil in ["", ".", "..", "a/b", "../etc", "x\0y"] {
+            assert!(
+                join_user_segment(Path::new("/tmp/x"), evil).is_err(),
+                "{evil:?} should be rejected"
+            );
+        }
+    }
+}