Function multibuffer_expand_excerpt_at 

pub fn multibuffer_expand_excerpt_at(
    services: &ServiceRegistry,
    buffer_id: BufferId,
    cursor_row: u32,
    delta: i32,
)

M.10.4 (2026-06-03): host glue for the :multibuffer-expand [n] / :multibuffer-contract [n] ex-commands. Looks up the active buffer’s typed MultibufferDocumentHandle via the MultibufferRegistryHandle service and calls expand_excerpt_at(cursor_row, delta). No-op when the active buffer isn’t a multibuffer view, the service isn’t registered (test harness), or the cursor is out of range.

Replaces Editor::do_multibuffer_expand which used to live in lattice-host::dispatch. Per [[feedback_mode_owns_its_surface]] + mode-architecture.md §5.3.4: this helper is the substrate-side counterpart to the ex-command registration in MultibufferMode. Host’s apply_effect arm calls this directly — no longer trampolines through Action::MultibufferExpand + Editor::do_multibuffer_expand.