core: avoid copying OSC 52 clipboard responses

OSC 52 clipboard reads built their response in an allocated buffer and
then passed it through Message.writeReq, which allocated a second copy
for large responses.

Instead, transfer the allocated response directly using .write_alloc.

Small responses now retain their initial allocation until the IO thread
consumes them instead of being copied inline and freed immediately.
Their allocation count is unchanged, while large responses improve from
two allocations to one. Both cases avoid the additional copy.

src/Surface.zig

@@ -5974,8 +5974,8 @@ fn completeClipboardReadOSC52(
     // This must hold the base64 encoded data PLUS the OSC code surrounding it.
     const enc = std.base64.standard.Encoder;
     const size = enc.calcSize(data.len);
-    var buf = try self.alloc.alloc(u8, size + 9); // const for OSC
-    defer self.alloc.free(buf);
+    const buf = try self.alloc.alloc(u8, size + 9); // const for OSC
+    errdefer self.alloc.free(buf);
 
     const kind: u8 = switch (clipboard_type) {
         .standard => 'c',
@@ -5993,10 +5993,10 @@ fn completeClipboardReadOSC52(
     const encoded = enc.encode(buf[prefix.len..], data);
     assert(encoded.len == size);
 
-    self.queueIo(try termio.Message.writeReq(
-        self.alloc,
-        buf,
-    ), .unlocked);
+    self.queueIo(.{ .write_alloc = .{
+        .alloc = self.alloc,
+        .data = buf,
+    } }, .unlocked);
 }
 
 fn showDesktopNotification(self: *Surface, title: [:0]const u8, body: [:0]const u8) !void {