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/src/books.rs

@@ -324,6 +324,9 @@ pub struct Books {
     pub statements: Vec<Statement>,
     /// Reconciliations, voided ones included, by date then id.
     pub reconciliations: Vec<Reconciliation>,
+    /// The format upgrade the books wait for, if they do (plan 3.9, D-39):
+    /// read from their journal, as the server reads it.
+    pub format_upgrade: Option<ledger_store::FormatUpgrade>,
 }
 
 /// A span of dates a party's money is summed over (the party page's and
@@ -838,6 +841,7 @@ impl Books {
             links,
             statements,
             reconciliations,
+            format_upgrade: snap.journal_format().upgrade(),
         };
         books.parties = books.party_infos();
         books.kpis = books.compute_kpis();