Restore directory navigation on Android back

EXPLORER.md

@@ -208,17 +208,18 @@ 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 back closes it first
+### 10. The explorer draws over the session, and the two back controls differ
 
 `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. Back — the button and the platform gesture — returns from an
-open file to its containing directory, then clears `files` and returns to the
-session. Directory navigation stays in the listing: `..` is an explicit row
-rather than a hidden second meaning for Back. An editor with unsaved changes
-asks before returning to the directory. "Back returns; it does not exit."
+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."
 
 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