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

Cargo.toml

@@ -19,3 +19,8 @@ anyhow = "1"
 drm = "0.12"
 drm-fourcc = "2"
 libc = "0.2"
+ashpd = { version = "0.13", features = ["tokio", "screencast"] }
+tokio = { version = "1", features = ["rt"] }
+pipewire = "0.9"
+libspa = "0.9"
+crossbeam-channel = "0.5"