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-host/src/lib.rs

@@ -0,0 +1,38 @@
+//! The books open while something holds them (plan 3.9, D-21, D-23).
+//!
+//! Nothing runs in the background: the first holder (the app, an agent's
+//! bridge, `wanderbooks serve`, or a command that needs the books) starts
+//! the **host**, which starts the books server and holds it open while
+//! any lease on its socket lasts, and closes it 10 s after the last one
+//! ends. Each part is a mode of the one `wanderbooks` executable:
+//! [`host::run`] (`wanderbooks host`), [`helper::run`] (`wanderbooks
+//! helper`), [`bridge::run`] (`wanderbooks mcp`), and the commands in
+//! [`commands`] (`serve`, `close`, `books`).
+
+#[cfg(unix)]
+pub mod bridge;
+#[cfg(unix)]
+pub mod commands;
+#[cfg(unix)]
+pub mod helper;
+#[cfg(unix)]
+pub mod host;
+pub mod knobs;
+pub mod lease;
+#[cfg(unix)]
+pub mod opener;
+#[cfg(unix)]
+mod start;
+
+/// How a mode stops when it can't go on: the reason is already printed,
+/// and the number is the process's exit code.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct Exit(pub i32);
+
+impl std::fmt::Display for Exit {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "exit code {}", self.0)
+    }
+}
+
+impl std::error::Error for Exit {}