feat(multicursor): MC HAMMER #41587

Other (squashed) commits:

fix(tui): emit ui_send output atomically with the frame

Problem:
tui_ui_send() writes directly to the TTY, bypassing the output buffer.
Sequences sent via nvim_ui_send() (e.g. kitty multiple-cursors, or
visual-dot-repeat) always arrive in a separate TTY write from the frame
they were computed for. This manifests as "tearing", or e.g. in the case
of multicursor the terminal renders text with stale cursor overlays.

Solution:
- tui_ui_send(): while a frame is being assembled (pending invalid
  regions or buffered output), buffer instead of writing directly.
  - Out-of-frame sends (tty queries, clear-on-disable) still write
    immediately.
- mcursor.lua: emit the terminal-cursor update at the end of the redraw
  cycle (`on_end`, when screen positions are final) instead of
  vim.schedule().

runtime/lua/vim/hl.lua

@@ -198,7 +198,9 @@ function M.hl_op(opts)
   local winid = api.nvim_get_current_win()
 
   local state = hl_op_state[state_key]
-  if state ~= nil and state.timer and not state.timer:is_closing() then
+  -- Multicursor cascade: accumulate per-cursor, don't cancel the previous event's highlight.
+  local cascading = api.nvim__mcursor_cascading()
+  if state ~= nil and state.timer and not state.timer:is_closing() and not cascading then
     state.timer:close()
     assert(state.clear)
     state.clear()