Outcome of TryRegisterBrowserPaneLive. Returned via
DispatchOutput::browser_pane_register_result. Same three-way semantics as the
pre-Phase-H pane::lifecycle::PaneStateMachine::try_register_live returned
β caller decides whether to start a fresh CEF create, re-navigate the
existing browser, or reject.
Capacity of TopLevelCreationState.history ring buffer. Configurable
via ~/.agentmux/config.toml [host.reducer] once H.5 (config) lands;
hard-coded for PR #1.