core: avoid allocating for pwd change actions (#13654)

Surface.handleMessage allocated a null-terminated copy for every working
directory update. OSC 7 values fit within the parser's 2 KiB fixed
buffer, so use stack-fallback storage sized for that bound and its
terminator.

The message type does not enforce the OSC bound, so an oversized future
producer still falls back to the heap. performAction already borrows the
value only for the duration of the call, preserving its existing
lifetime.

src/Surface.zig

@@ -1073,9 +1073,10 @@ pub fn handleMessage(self: *Surface, msg: Message) !void {
         .pwd_change => |w| {
             defer w.deinit();
 
-            // We always allocate for this because we need to null-terminate.
-            const str = try self.alloc.dupeZ(u8, w.slice());
-            defer self.alloc.free(str);
+            var stack = std.heap.stackFallback(256, self.alloc);
+            const alloc = stack.get();
+            const str = try alloc.dupeZ(u8, w.slice());
+            defer alloc.free(str);
 
             _ = try self.rt_app.performAction(
                 .{ .surface = self },