CI: ship the tray GUI — in the archives and as tsunagi-gui deb/zst

Adds the tray application to the release:

- Every target's archive now also contains the tsunagi-tray binary. On Linux it
  is built against an old glibc baseline (via cargo-zigbuild) rather than static
  musl, because a GUI must dlopen libGL/X11/Wayland at runtime and a static musl
  binary cannot; the agent stays static musl. macOS and Windows build the tray
  natively.
- New GUI package variants for Linux: `tsunagi-gui` .deb and .pkg.tar.zst, built
  by the existing scripts with an extra tray argument. They install the agent
  (service, polkit, user — same as the headless packages) plus the tray binary,
  a desktop launcher (dist/linux/tsunagi-tray.desktop) and an icon
  (dist/linux/tsunagi.svg), so the GUI appears in the applications menu. The GUI
  package conflicts/replaces/provides the headless tsunagi so the two do not
  coexist.
- The tray finds the system service with no configuration: the library's default
  control-socket path now prefers /run/tsunagi/agent.sock when present (both the
  tray and CLI use this), falling back to the per-user path.

No control-protocol change. Builds, fmt and clippy clean on macOS; the Linux GUI
cross-build and the packages are exercised only by a real CI run.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dist/linux/tsunagi-tray.desktop

@@ -0,0 +1,11 @@
+[Desktop Entry]
+Type=Application
+Name=tsunagi
+GenericName=Mesh network manager
+Comment=Manage tsunagi networks from the tray
+Exec=tsunagi-tray
+Icon=tsunagi
+Terminal=false
+Categories=Network;Utility;
+Keywords=mesh;vpn;overlay;network;
+StartupNotify=false

dist/linux/tsunagi.svg

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="128" height="128" viewBox="0 0 128 128">
+  <circle cx="64" cy="64" r="56" fill="#2f80d8"/>
+  <circle cx="64" cy="64" r="30" fill="none" stroke="#ffffff" stroke-width="7" opacity="0.9"/>
+  <circle cx="64" cy="64" r="9" fill="#ffffff"/>
+</svg>