fix(client-linux): absolute mouse was dropped — pack the surface size in flags

The MouseMoveAbs wire contract packs the client coordinate-space size
as (width << 16) | height in `flags` (same as touch); injectors
normalize against it and drop the event when it is zero. The GTK
client sent flags=0, so KWin's libei path refused every motion
(`emitted=false`) — found via the first real user test from
home-worker-3.

- ui_stream: send_abs() packs the negotiated mode into flags for
  motion + click-position events.
- core input.rs: document the contract on MouseMoveAbs itself (it was
  only implied by TouchDown's doc).
- client-rs --input-test: add a MouseMoveAbs sweep so the absolute
  path stays covered — Moonlight and the Mac client only send relative
  motion, which is why this gap survived every prior live test.

Validated live against serve --native: kind=MouseMoveAbs emitted=true.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-client-rs/src/main.rs

@@ -528,6 +528,7 @@ async fn session(args: Args) -> Result<()> {
     // low-latency input path without a real input device.
     if args.input_test {
         let conn2 = conn.clone();
+        let (mw, mh) = (args.mode.width, args.mode.height);
         tokio::spawn(async move {
             tokio::time::sleep(std::time::Duration::from_secs(2)).await;
             tracing::info!("input-test: sending scripted datagrams for ~6s");
@@ -547,6 +548,17 @@ async fn session(args: Args) -> Result<()> {
                     flags: 0,
                 };
                 let _ = conn2.send_datagram(mv.encode().to_vec().into());
+                // Absolute motion too (the GTK client's path): a diagonal sweep, with the
+                // coordinate-space size packed in `flags` — the contract injectors require.
+                let abs = InputEvent {
+                    kind: InputKind::MouseMoveAbs,
+                    _pad: [0; 3],
+                    code: 0,
+                    x: ((i * mw) / 160) as i32,
+                    y: ((i * mh) / 160) as i32,
+                    flags: (mw << 16) | (mh & 0xffff),
+                };
+                let _ = conn2.send_datagram(abs.encode().to_vec().into());
                 if i % 20 == 0 {
                     for kind in [InputKind::KeyDown, InputKind::KeyUp] {
                         let key = InputEvent {