feat(vdisplay): wlroots/Sway backend — swaymsg headless output + xdpw chooser

The fourth VirtualDisplay backend: `swaymsg create_output` adds a HEADLESS-N
output (name found by diffing get_outputs), `output <NAME> mode --custom
WxH@HzHz` sets the client's exact mode (and the refresh clock a fresh headless
output needs to produce frames at all), and the PipeWire node comes from the
ScreenCast portal. Headless output selection is non-interactive via
xdg-desktop-portal-wlr's chooser hook: a managed config (chooser_type=simple,
chooser_cmd cats /tmp/punktfunk-xdpw-output; portal try-restarted when the
config changes) plus a per-session `Monitor: <NAME>` written to that file.
Teardown is RAII: drop ends the portal thread (zbus connection drop ends the
cast) then `swaymsg output <NAME> unplug`. swaymsg commands go after `--` so
tokens like `--custom` reach sway instead of swaymsg's getopt.

Validated live on headless sway 1.11 (gles2-on-NVIDIA, xdpw 0.8.1), zero-copy
dmabuf→CUDA on both runs: 720p60 257 frames p50 0.77 ms, 1080p60 480/480
frames p50 1.18 ms, output unplugged with the session both times. The
checked-in xdpw.config sample now matches the managed config (the old
chooser_type=none/HEADLESS-1 form would pin capture to the wrong output).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/headless/xdpw.config

@@ -1,6 +1,4 @@
-# ~/.config/xdg-desktop-portal-wlr/config
-# Headless ScreenCast: there is no GUI output chooser, so select the output by name.
+# managed by punktfunk (vdisplay/wlroots.rs) — per-session output selection.
 [screencast]
-chooser_type=none
-output_name=HEADLESS-1
-max_fps=60
+chooser_type=simple
+chooser_cmd=cat /tmp/punktfunk-xdpw-output 2>/dev/null || echo 'Monitor: HEADLESS-1'