feat: add readonly surface mode (#9130)

Tried my hand at #8432

Currently lacking tests and some sort of visual indicator that the
surface is locked.

Also, not entirely sure if I needed to touch `application.zig`.

Found what needed changing with help from Copilot (and added Docs w/
Copilot), but wrote most of the code myself.

src/termio/Termio.zig

@@ -22,6 +22,9 @@ const configpkg = @import("../config.zig");
 
 const log = std.log.scoped(.io_exec);
 
+/// Mutex state argument for queueMessage.
+pub const MutexState = enum { locked, unlocked };
+
 /// Allocator
 alloc: Allocator,
 
@@ -380,7 +383,7 @@ pub fn threadExit(self: *Termio, data: *ThreadData) void {
 pub fn queueMessage(
     self: *Termio,
     msg: termio.Message,
-    mutex: enum { locked, unlocked },
+    mutex: MutexState,
 ) void {
     self.mailbox.send(msg, switch (mutex) {
         .locked => self.renderer_state.mutex,