core: Acquire renderer state mutex before calling processLinks (ghostty-org#12463)

Holding the renderer state mutex is a documented precondition of
`processLinks`, but `mouseButtonCallback` previously called the function
without the mutex.

This creates a race with the I/O thread's `processOutput`, which can
prune scrollback pages while `processLinks` is reading them, resulting
in a use-after-free segfault. See
ghostty-org#12409 (Linux: crash
while selecting text).


https://github.com/ghostty-org/ghostty/blob/57b5e1e2507cd65ab8197d39baa4ce2505185510/src/Surface.zig#L4354-L4355


https://github.com/ghostty-org/ghostty/blob/57b5e1e2507cd65ab8197d39baa4ce2505185510/src/Surface.zig#L3822-L3824

995e4e3 (os: open) changed the body of `processLinks` to be
non-trivial and documented the precondition, but the lock was not held
at the call site.

Author: mitchellh

src/Surface.zig

@@ -3821,6 +3821,8 @@ pub fn mouseButtonCallback(
         // clicked link will swallow the event.
         if (self.mouse.over_link) {
             const pos = try self.rt_surface.getCursorPos();
+            self.renderer_state.mutex.lock();
+            defer self.renderer_state.mutex.unlock();
             if (self.processLinks(pos)) |processed| {
                 if (processed) return true;
             } else |err| {