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/Cargo.toml

@@ -0,0 +1,27 @@
+[package]
+name = "ledger-host"
+version = "0.1.0"
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+description = "wanderbooks: the books open while something holds them (the host, its leases, the helper and the agents' bridge)."
+
+[dependencies]
+ledger-core = { path = "../ledger-core" }
+ledger-mcp = { path = "../ledger-mcp" }
+ledger-store = { path = "../ledger-store" }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "io-util", "io-std", "sync", "net", "signal", "time", "process"] }
+reqwest = "0.13"
+base64 = "0.22"
+tokio-util = { version = "0.7", features = ["rt"] }
+
+[target.'cfg(unix)'.dependencies]
+libc = "0.2"
+
+[dev-dependencies]
+tempfile = "3"
+
+[lints]
+workspace = true