PLAN: record the std.fs directory-listing gap (cleanup.sx shims readdir)

PLAN.md

@@ -183,6 +183,8 @@ subplan or has an explicit local workaround:
   (byte length, scalar values, grapheme clusters, and display width are
   distinct; invalid UTF-8 must not silently become a `String`).
 - Bytes and a full path module (only `path_join`/`basename`/`dirname` today).
+- `std.fs` directory listing (`src/store/cleanup.sx` shims
+  opendir/readdir/closedir via FFI meanwhile).
 - Time/clock (publish shims `time(2)` via FFI), random, encodings
   (base64url, percent).
 - HTTP server/client and TLS boundary — blocks subplan 04 and remote publish