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_state/src/lib.rs

@@ -187,6 +187,10 @@ fn normalize_snapshot(snapshot: ServerSnapshot) -> ServerSnapshot {
     }
 }
 
+// TODO(refactor): StateEvent and Delta have mirrored variants (e.g.
+// StateEvent::ChannelChanged/ChannelDeleted vs Delta::ChannelUpserted/ChannelRemoved).
+// A proc-macro or macro_rules could generate the Delta-from-StateEvent mapping, but
+// the manual match is currently clear and the types serve different roles (input vs output).
 /// A change to the server state that the bridge should publish to
 /// Flutter. Deltas are cheap to construct and carry only the
 /// information that changed.