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/gamescope.rs

@@ -198,6 +198,17 @@ fn find_gamescope_node() -> Option<u32> {
     None
 }
 
+/// gamescope is usable wherever its binary runs — it spawns its own nested session, so it does
+/// not require any particular desktop to be running. Quiet (no version warning — that's for the
+/// create path); just checks the binary executes.
+pub fn is_available() -> bool {
+    std::process::Command::new("gamescope")
+        .arg("--version")
+        .output()
+        .map(|o| o.status.success())
+        .unwrap_or(false)
+}
+
 /// Minimum gamescope that captures reliably: below 3.16.22, headless PipeWire capture deadlocks
 /// against PipeWire ≥ 1.6 (a loop-lock bug) and a stuck link head-blocks the whole daemon.
 const MIN_GAMESCOPE: (u32, u32, u32) = (3, 16, 22);
@@ -255,7 +266,10 @@ mod tests {
 
     #[test]
     fn parses_version_banner() {
-        assert_eq!(parse_version("gamescope version 3.16.22"), Some((3, 16, 22)));
+        assert_eq!(
+            parse_version("gamescope version 3.16.22"),
+            Some((3, 16, 22))
+        );
         assert_eq!(
             parse_version("gamescope: version v3.15.9 (no PipeWire)"),
             Some((3, 15, 9))