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/api/ui_spec.lua

@@ -133,9 +133,10 @@ describe('nvim_ui_send', function()
 
     screen:expect_unchanged()
 
-    -- The TUI client queries OSC 11 on connect, so that precedes the payload.
-    local bg_request = '\027]11;?\007'
-    eq(bg_request .. 'Hello world', table.concat(read_data))
+    -- On connect, these queries precede the payload.
+    local mcursor_request = '\027[> q' -- kitty-multicursor (CSI > SP q) query.
+    local bg_request = '\027]11;?\007' -- TUI client OSC 11 query.
+    eq(mcursor_request .. bg_request .. 'Hello world', table.concat(read_data))
   end)
 
   it('ignores ui_send event for UIs without stdout_tty', function()