feat: add KWin/KDE Plasma screen capture via xdg-desktop-portal ScreenCast + PipeWire

Add a second capture backend for compositors without wlr-screencopy
(KWin, GNOME, etc.) using the xdg-desktop-portal ScreenCast interface
and PipeWire DMA-BUF streaming.

New files:
- src/backend_detect.rs: auto-detect wlr-screencopy vs portal backend
- src/cap_portal.rs: Portal session setup + PipeWire DMA-BUF thread
- src/state_portal.rs: StatePortal encoder pipeline (DMA-BUF → VAAPI)

Changes:
- Cargo.toml: add ashpd 0.13, tokio 1, pipewire 0.9, libspa 0.9,
  crossbeam-channel 0.5
- src/args.rs: add --backend CLI flag
- src/avhw.rs: extract create_encoder() from inline State code
- src/main.rs: route to portal or wlr-screencopy based on backend
- src/state.rs: fix params.destroy() on dup failure, cleanup
  in_flight_surface on copy fail, use create_encoder()
- tests/integration_test.rs: add --backend flag tests

tests/integration_test.rs

@@ -44,10 +44,7 @@ fn test_rejects_invalid_args() {
         .output()
         .expect("failed to execute wl-webrtc with invalid args");
 
-    assert!(
-        !output.status.success(),
-        "should reject unrecognized flag"
-    );
+    assert!(!output.status.success(), "should reject unrecognized flag");
     let stderr = String::from_utf8_lossy(&output.stderr);
     assert!(
         stderr.to_lowercase().contains("error")
@@ -68,10 +65,7 @@ fn test_rejects_hevc_codec() {
         .expect("failed to execute wl-webrtc --codec hevc");
 
     // MVP only supports h264; hevc should be rejected.
-    assert!(
-        !output.status.success(),
-        "should reject hevc codec in MVP"
-    );
+    assert!(!output.status.success(), "should reject hevc codec in MVP");
 }
 
 /// Tests requiring a live Wayland compositor and VAAPI hardware.