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

Cargo.toml

@@ -1,6 +1,6 @@
 [workspace]
 resolver = "2"
-members = ["crates/ledger-core", "crates/ledger-store", "crates/ledger-config", "crates/ledger-mcp", "crates/ledger-import", "crates/ledger-cli", "crates/ledger-gui", "crates/ledger-vault"]
+members = ["crates/ledger-core", "crates/ledger-store", "crates/ledger-config", "crates/ledger-mcp", "crates/ledger-import", "crates/ledger-cli", "crates/ledger-gui", "crates/ledger-vault", "crates/ledger-host"]
 
 [workspace.package]
 edition = "2024"