fix(lifecycle): on Windows, CTRL_CLOSE kills Nvim mid-teardown

Problem:
`signal_ignore_deadly` doesn't work for Windows, where the console still
may terminate Nvim during teardown (after `signal_teardown`), while it
is already trying to exit. Besides interrupting any housekeeping we are
doing, it results in an unpredictable exit code (flaky tests).

    [Process exited -1073741510]  // 0xC000013A STATUS_CONTROL_C_EXIT

Solution:
Register our own CTRL_CLOSE_EVENT handler which "blocks" the signal.
Note: if exit takes longer than 5s, Windows will consider the process
"hung" and kill it anyway.

src/nvim/os/signal.c

@@ -66,14 +66,32 @@ void signal_init(void)
   signal_start();
 }
 
+#ifdef MSWIN
+/// Swallows console control events during shutdown.
+static BOOL WINAPI signal_ctrl_handler(DWORD type)
+{
+  switch (type) {
+  case CTRL_CLOSE_EVENT:
+    // Returning true is not enough; Windows terminates the process as soon as the handler returns.
+    // So block instead, like libuv's own handler does, and let the exit finish.
+    Sleep(INFINITE);
+    return true;
+  case CTRL_C_EVENT:
+  case CTRL_BREAK_EVENT:
+    return true;
+  default:
+    return false;
+  }
+}
+#endif
+
 /// During shutdown, we don't want the default actions of these signals.
-///
-/// Note: Windows still has the race. libuv's console control handler dispatches CTRL_CLOSE_EVENT
-/// as SIGHUP, but only while a watcher is registered; signal_teardown() closed ours, so the
-/// console terminates Nvim with STATUS_CONTROL_C_EXIT (-1073741510).
 static void signal_ignore_deadly(void)
 {
-#ifndef MSWIN
+#ifdef MSWIN
+  // Handlers run in reverse registration order, so this one runs before libuv's.
+  SetConsoleCtrlHandler(signal_ctrl_handler, true);
+#else
   signal(SIGHUP, SIG_IGN);
   signal(SIGINT, SIG_IGN);
   signal(SIGTERM, SIG_IGN);

test/functional/terminal/tui_spec.lua

@@ -3669,10 +3669,7 @@ describe('TUI', function()
     retry(nil, 2000, function()
       eq(vim.NIL, api.nvim_get_proc(pid))
     end)
-    -- On Windows the console terminates the child with STATUS_CONTROL_C_EXIT (-1073741510).
-    screen:expect({
-      any = is_os('win') and '%[Process exited %-?%d+%]' or '%[Process exited 1%]',
-    })
+    screen:expect({ any = '%[Process exited 1%]' })
     -- Closing stdin must skip the DA1 wait.
     t.assert_nolog('timed out waiting for DA1 response', testlog, 100)
   end)