feat(lbm-agent): the connection to the hook, and a fake hook to develop against

[mgth]

Phase 3 of the v6 plan, next brick after the reconciler (#623): the agent's connection to the hook, and a fake hook to develop and test it without capturing anyone's mice.

lbm-ipc

• endpoint: where the hook listens — the Linux socket ($XDG_RUNTIME_DIR/littlebigmouse-v1.sock, else in the data dir), the per-session Windows pipe, LBM_HOOK_ENDPOINT — one definition for the daemon and its clients. lbm-hook now takes its default endpoint from it: same names as before, no behaviour change.
• client::listen is spelled as the C# LocalIpcClient writes it; client::messages batches commands in one <Messages> frame, as C# sends every batch — Load and Run together keep the hook up across a re-apply (frame_rehooks).

lbm-agent

• hook::HookClient: one connection for both directions — subscribe (Listen), then events in and commands out, in order. (C# opened a fresh connection per command, which is one of the ways commands could reach the daemon out of order — Cursor gets trapped at screen edges after disconnecting and waking from sleep #607/fix: never hook a layout the desktop no longer matches (#607) #609.) Kept up and retried every 100 ms; Lost when it drops (C# synthesizes Dead there), Unreachable on the first failed attempt then about every 5 s (C#'s ConnectionFailed cadence, the cue for launching a hook). Commands given while disconnected are dropped, never replayed stale — a reconnected hook asks for its layout anyway (Connected → Stopped, answered by the reconciler).
• fake_hook::FakeHook: speaks the protocol and answers as lbm-hook does (state on Listen, Loaded/LoadFailed, Running/Stopped, Load unhooks unless its frame also runs), touching no input device. What --fake-hook will run, and what the tests talk to over a private endpoint.

Tests

Against the fake hook over a real Unix socket (a named pipe on the Windows runner): subscription and state, Load+Run in one frame, the hook's own events, loss and reconnection with nothing replayed, and the unreachable cadence. Repeated 15 times locally without a failure.

Nothing launches or talks to the real hook yet: supervision (D5) and the runtime come next.

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