Return from files to explorer

EXPLORER.md

@@ -208,18 +208,19 @@ absence the signal. Back with unsaved changes asks, and says the edits will
 be lost. The explorer draws over the session, which deliberately has no
 `imePadding`, so the explorer's own box adds it.
 
-### 10. The explorer draws over the session, and the two back controls differ
+### 10. The explorer draws over the session, and back follows what is open
 
 `Screen.Session` in `AppRoot` gains a `files: FilesTarget?`. When set, the
 `FilesScreen` is composed **on top of** the session in the same `Box`, and
 the session stays composed under it: its event stream keeps flowing, its
 scroll position and draft stay where they were, and returning from a file
-costs nothing. The header's back button clears `files` and returns directly to
-the session. Android back returns from an open file to its containing directory,
-then walks to the parent directory until the root; at the root it returns to the
-session. The `..` row remains as the visible, tappable form of the same directory
-movement. An editor with unsaved changes asks before either route discards them.
-"Back returns; it does not exit."
+costs nothing. From an open file, both the header's back button and Android back
+return to its containing directory. From a directory, the header's back button
+clears `files` and returns to the session, while Android back walks to the parent
+directory until the root and only then returns to the session. The `..` row
+remains as the visible, tappable form of the same directory movement. An editor
+with unsaved changes asks before either route discards them. "Back returns; it
+does not exit."
 
 Rejected: a `Screen.Files` beside `Screen.Session`. Every route back from a
 leaf screen goes to Main today, and a session disposed and re-created on each