P3: the books' format moves only with the owner's go-ahead (D-39)

Books an update could write a newer format for open on their old format
and are asked at every open; anything needing a new feature is refused
until the owner agrees with the passphrase, after a verified backup that
includes the keys. New books start on the current format. Each projection
schema has its own database file, so an older version still opens the
books until the owner agrees.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-gui/tests/owner_menu.rs

@@ -74,6 +74,10 @@ impl Owner for Fake {
         self.said(format!("connect {agent:?} {book}"));
         Ok(vec!["Claude Code reaches the books.".into()])
     }
+    fn upgrade_format(&self, book: &str, _passphrase: &str) -> Result<Vec<String>, String> {
+        self.said(format!("upgrade {book}"));
+        Err("not in these tests".into())
+    }
 }
 
 /// One test at a time: they share the one owner and its record of calls.