P2 slice 1: one executable (D-22)

The server and the viewer are libraries with run(prog, args), returning
Exit(code) instead of ending the process; ledger-mcp and ledger-gui are
one-line shims (the live books keep using them until the cutover).
wanderbooks dispatches dev-stdio and gui to them in-process; a mode must
come first and owns its flags. The gui feature (default) leaves the
viewer out of the server-only build, which just server-only builds,
clippies, tests, and checks for window libraries. The viewer's mutation
recipe is retired (owner, 2026-10-18); its two musts are tested
directly. Two review rounds; mutation on the diff (docs/testing.md).

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

Cargo.lock

@@ -3655,6 +3655,8 @@ dependencies = [
  "chrono",
  "ledger-config",
  "ledger-core",
+ "ledger-gui",
+ "ledger-mcp",
  "serde_json",
  "tempfile",
  "toml 1.1.8+spec-1.1.0",