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().

test/functional/lua/ui_event_spec.lua

@@ -452,7 +452,7 @@ describe('vim.ui_attach', function()
         end
       end)
     ]])
-    feed('Q')
+    feed('@@')
     screen:expect({
       grid = [[
                                                                           |
@@ -479,7 +479,7 @@ describe('vim.ui_attach', function()
         end
       end)
     ]])
-    feed('Q')
+    feed('@@')
     screen:expect({
       grid = [[
                                                                           |