P2 slice 3: opening and closing the books (plan 3.9, D-21, D-23)

One wanderbooks executable. The books are open while the viewer, an
agent session or a terminal holds them: a host process takes leases on
a socket, starts the server (the hidden mode `wanderbooks server`) with
a pipe on its input, and closes it 10 s after the last lease ends. No
background service. One host even in a race (run/host.lock).

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

crates/ledger-store/src/store.rs

@@ -6235,7 +6235,7 @@ fn row_to_attachment(r: &rusqlite::Row) -> rusqlite::Result<Attachment> {
 
 /// The base currency of a journal: its first line's, which must be a
 /// sound `store.created` ([`created_currency`]).
-fn journal_currency(lines: &[JournalLine]) -> Result<Currency, StoreError> {
+pub(crate) fn journal_currency(lines: &[JournalLine]) -> Result<Currency, StoreError> {
     let first = lines
         .first()
         .ok_or_else(|| StoreError::JournalCorrupt("journal has no store.created line".into()))?;