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>

include/punktfunk_core.h

@@ -273,7 +273,10 @@ enum PunktfunkInputKind
     PUNKTFUNK_INPUT_KIND_KEY_UP = 1,
     // Relative motion: `x`/`y` carry `dx`/`dy`.
     PUNKTFUNK_INPUT_KIND_MOUSE_MOVE = 2,
-    // Absolute motion: `x`/`y` carry pixel coordinates.
+    // Absolute motion: `x`/`y` carry pixel coordinates and `flags` packs the client's
+    // coordinate-space size as `(width << 16) | height` (the same contract as
+    // [`TouchDown`](Self::TouchDown)) — injectors normalize against it before mapping
+    // into the output region and **drop the event when it is zero**.
     PUNKTFUNK_INPUT_KIND_MOUSE_MOVE_ABS = 3,
     PUNKTFUNK_INPUT_KIND_MOUSE_BUTTON_DOWN = 4,
     PUNKTFUNK_INPUT_KIND_MOUSE_BUTTON_UP = 5,