refactor(core,storage): deduplicate code patterns (TODO-014)

Extract PttCapability event construction helper in events.rs.
Extract ensure_dir() helper in chanora_storage. Add TODO(refactor)
annotations for patterns requiring shared crate or API changes.

crates/chanora_storage/src/lib.rs

@@ -79,6 +79,10 @@ pub enum StorageError {
     Crypto(String),
 }
 
+fn ensure_dir(dir: &Path) -> Result<(), StorageError> {
+    fs::create_dir_all(dir).map_err(|e| StorageError::Io(format!("mkdir {dir:?}: {e}")))
+}
+
 /// Audio-related per-identity settings persisted alongside the
 /// identity file as a small JSON blob (SDD-095 / SDD-096). These
 /// are *not* secrets; they sit beside the encrypted identity in
@@ -193,7 +197,7 @@ impl IdentityFileStore {
     /// the DEK on first use; subsequent uses reuse the existing DEK.
     pub fn new(dir: impl AsRef<Path>) -> Result<Self, StorageError> {
         let dir = dir.as_ref();
-        fs::create_dir_all(dir).map_err(|e| StorageError::Io(format!("mkdir {dir:?}: {e}")))?;
+        ensure_dir(dir)?;
         let canonical = fs::canonicalize(dir)
             .map(|p| p.to_string_lossy().into_owned())
             .unwrap_or_else(|_| dir.to_string_lossy().into_owned());
@@ -814,7 +818,7 @@ impl BookmarkRepository {
     }
 
     fn open(dir: &Path, crypto: Option<Box<dyn Crypto>>) -> Result<Self, StorageError> {
-        fs::create_dir_all(dir).map_err(|e| StorageError::Io(format!("mkdir {dir:?}: {e}")))?;
+        ensure_dir(dir)?;
         let path = dir.join("chanora.db");
         let conn = Connection::open(&path)
             .map_err(|e| StorageError::Sqlite(format!("open {path:?}: {e}")))?;