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-mcp/src/lib.rs

@@ -5,8 +5,9 @@
 //!
 //! Usage: `ledger-mcp [--data-dir PATH] [--config PATH] [--books-toml PATH]`
 //!
-//! The data dir holds `journal.jsonl` (the source of truth), `wanderbooks.db`
-//! (the disposable SQLite projection), `blobs/` (content-addressed receipts),
+//! The data dir holds `journal.jsonl` (the source of truth),
+//! `wanderbooks.v{N}.db` (the disposable SQLite projection, a file for each
+//! schema: ledger_store::db_file_for), `blobs/` (content-addressed receipts),
 //! and optionally `config.json`. If no store exists, one is created with the
 //! configured base currency. Company settings (name, fiscal year start,
 //! receipt threshold) come from the books dir's `wanderbooks.toml`, by