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>

web/src/routes/host.tsx

@@ -1,5 +1,5 @@
 import { createFileRoute } from '@tanstack/react-router'
-import { useGetHostInfo } from '@/api/gen/host/host'
+import { useGetHostInfo, useListCompositors } from '@/api/gen/host/host'
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card'
 import { Badge } from '@/components/ui/badge'
 import { QueryState } from '@/components/query-state'
@@ -11,6 +11,7 @@ export const Route = createFileRoute('/host')({ component: HostPage })
 function HostPage() {
   useLocale()
   const host = useGetHostInfo()
+  const compositors = useListCompositors()
   const h = host.data
 
   return (
@@ -65,6 +66,37 @@ function HostPage() {
           </div>
         )}
       </QueryState>
+
+      <Card>
+        <CardHeader>
+          <CardTitle>{m.host_compositors()}</CardTitle>
+        </CardHeader>
+        <CardContent className="space-y-4">
+          <p className="text-sm text-muted-foreground">{m.host_compositors_help()}</p>
+          <QueryState
+            isLoading={compositors.isLoading}
+            error={compositors.error}
+            refetch={compositors.refetch}
+          >
+            <ul className="divide-y rounded-md border">
+              {compositors.data?.map((c) => (
+                <li key={c.id} className="flex items-center justify-between gap-4 px-4 py-3">
+                  <div className="min-w-0">
+                    <div className="flex items-center gap-2">
+                      <span className="font-medium">{c.label}</span>
+                      {c.default && <Badge variant="secondary">{m.compositor_default()}</Badge>}
+                    </div>
+                    <code className="text-xs text-muted-foreground">{c.id}</code>
+                  </div>
+                  <Badge variant={c.available ? 'default' : 'outline'}>
+                    {c.available ? m.compositor_available() : m.compositor_unavailable()}
+                  </Badge>
+                </li>
+              ))}
+            </ul>
+          </QueryState>
+        </CardContent>
+      </Card>
     </div>
   )
 }