feat: client-selectable compositor (protocol → host → client → C ABI → mgmt → web)

A client can now request which compositor backend the host drives its virtual
output on (gamescope/KWin/Mutter/wlroots). The host honors the request if that
backend is available, else falls back to auto-detect and reports the resolved
choice back — wire-compatible both directions (no ABI bump).

Protocol (punktfunk-core):
- New CompositorPref (config.rs): Auto|Kwin|Wlroots|Mutter|Gamescope with
  u8/name mappings. Appended as one optional byte to Hello (client preference)
  and Welcome (host's resolved choice). Both decoders already tolerate trailing
  bytes, so old↔new interop is preserved — ABI_VERSION stays 2. Round-trip +
  back-compat (truncated-message) tests.
- C ABI: punktfunk_connect_ex(compositor) + PUNKTFUNK_COMPOSITOR_* constants;
  punktfunk_connect delegates with AUTO, so the existing symbol is unchanged.
  NativeClient::connect / worker_main thread the preference through.

Host:
- vdisplay::available() enumerates usable backends via cheap, side-effect-free
  probes (KWin zkde global, gamescope binary+version, GNOME/Sway env), plus
  Compositor id/label/as_pref/from_pref/all helpers.
- m3 handshake resolves the preference to a concrete backend during the
  handshake (pick_compositor pure + resolved logging), reports it in Welcome,
  and threads it into virtual_stream (replacing the unconditional detect()).
- mgmt GET /v1/compositors lists every backend with availability + the
  auto-detected default (OpenAPI regenerated).

Client:
- punktfunk-client-rs --compositor NAME; logs the host's resolved choice from
  the Welcome ("session offer … compositor=…").

Web console:
- Host page gains a Compositors card (availability + default badges) via the
  codegen'd useListCompositors hook; en/de strings added.

Also fixes a pre-existing, env-dependent test-isolation bug:
mgmt::tests::paired_clients_list_and_unpair seeded the real
~/.config/punktfunk/paired.json (AppState::new loads it), so a real
GameStream-paired client leaked into body[0] on a dev box — now cleared first.

Live-validated against headless KWin: --compositor kwin honored, --compositor
mutter falls back to kwin (available=[kwin, gamescope]), resolved choice
round-trips to the client. Tests: +6 (wire/back-compat, resolution precedence,
endpoint); workspace green, clippy/fmt clean, C ABI harness PASS at abi_version=2,
web typecheck + build clean.

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

crates/punktfunk-host/src/vdisplay.rs

@@ -59,6 +59,91 @@ pub enum Compositor {
     Gamescope,
 }
 
+impl Compositor {
+    /// Stable lowercase id used on the wire / management API (matches
+    /// [`punktfunk_core::CompositorPref::as_str`]).
+    pub fn id(self) -> &'static str {
+        match self {
+            Compositor::Kwin => "kwin",
+            Compositor::Wlroots => "wlroots",
+            Compositor::Mutter => "mutter",
+            Compositor::Gamescope => "gamescope",
+        }
+    }
+
+    /// Human label for UIs.
+    pub fn label(self) -> &'static str {
+        match self {
+            Compositor::Kwin => "KWin / KDE Plasma",
+            Compositor::Wlroots => "wlroots (Sway / Hyprland)",
+            Compositor::Mutter => "Mutter / GNOME",
+            Compositor::Gamescope => "gamescope",
+        }
+    }
+
+    /// The protocol [`punktfunk_core::CompositorPref`] naming this backend.
+    pub fn as_pref(self) -> punktfunk_core::CompositorPref {
+        use punktfunk_core::CompositorPref as P;
+        match self {
+            Compositor::Kwin => P::Kwin,
+            Compositor::Wlroots => P::Wlroots,
+            Compositor::Mutter => P::Mutter,
+            Compositor::Gamescope => P::Gamescope,
+        }
+    }
+
+    /// The concrete backend a [`punktfunk_core::CompositorPref`] names, or `None` for `Auto`.
+    pub fn from_pref(p: punktfunk_core::CompositorPref) -> Option<Compositor> {
+        use punktfunk_core::CompositorPref as P;
+        Some(match p {
+            P::Auto => return None,
+            P::Kwin => Compositor::Kwin,
+            P::Wlroots => Compositor::Wlroots,
+            P::Mutter => Compositor::Mutter,
+            P::Gamescope => Compositor::Gamescope,
+        })
+    }
+
+    /// Every backend, in a stable display order (for enumeration / UIs).
+    pub fn all() -> [Compositor; 4] {
+        [
+            Compositor::Kwin,
+            Compositor::Gamescope,
+            Compositor::Mutter,
+            Compositor::Wlroots,
+        ]
+    }
+}
+
+/// The compositor backends usable on this host *right now*: gamescope wherever its binary is
+/// installed (it spawns a nested session — independent of the running desktop), plus the live
+/// session's own compositor (KWin / Mutter / wlroots) when the host runs inside it. Cheap,
+/// side-effect-free probes — safe to call per management request. A concrete client preference
+/// is validated against this set before it's honored (see the m3 handshake's resolution).
+pub fn available() -> Vec<Compositor> {
+    #[cfg(target_os = "linux")]
+    {
+        let mut v = Vec::new();
+        if kwin::is_available() {
+            v.push(Compositor::Kwin);
+        }
+        if gamescope::is_available() {
+            v.push(Compositor::Gamescope);
+        }
+        if mutter::is_available() {
+            v.push(Compositor::Mutter);
+        }
+        if wlroots::is_available() {
+            v.push(Compositor::Wlroots);
+        }
+        v
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        Vec::new()
+    }
+}
+
 /// Detect the compositor to drive: `PUNKTFUNK_COMPOSITOR` override, else `XDG_CURRENT_DESKTOP`.
 pub fn detect() -> Result<Compositor> {
     if let Ok(v) = std::env::var("PUNKTFUNK_COMPOSITOR") {