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-core/src/client.rs

@@ -11,7 +11,7 @@
 //! invariant) plus a blocking data-plane pump; frames cross to the embedder over a bounded
 //! channel. All methods are safe to call from any single embedder thread.
 
-use crate::config::{Mode, Role};
+use crate::config::{CompositorPref, Mode, Role};
 use crate::error::{PunktfunkError, Result};
 use crate::input::InputEvent;
 use crate::quic::{endpoint, io, Hello, Reconfigure, Reconfigured, Start, Welcome};
@@ -76,6 +76,7 @@ impl NativeClient {
         host: &str,
         port: u16,
         mode: Mode,
+        compositor: CompositorPref,
         pin: Option<[u8; 32]>,
         identity: Option<(String, String)>,
         timeout: Duration,
@@ -110,6 +111,7 @@ impl NativeClient {
                     host,
                     port,
                     mode,
+                    compositor,
                     pin,
                     identity,
                     frame_tx,
@@ -309,6 +311,7 @@ struct WorkerArgs {
     host: String,
     port: u16,
     mode: Mode,
+    compositor: CompositorPref,
     pin: Option<[u8; 32]>,
     identity: Option<(String, String)>,
     frame_tx: SyncSender<Frame>,
@@ -328,6 +331,7 @@ async fn worker_main(args: WorkerArgs) {
         host,
         port,
         mode,
+        compositor,
         pin,
         identity,
         frame_tx,
@@ -374,11 +378,18 @@ async fn worker_main(args: WorkerArgs) {
             &Hello {
                 abi_version: crate::ABI_VERSION,
                 mode,
+                compositor,
             }
             .encode(),
         )
         .await?;
         let welcome = Welcome::decode(&io::read_msg(&mut recv).await?)?;
+        if welcome.compositor != CompositorPref::Auto {
+            tracing::info!(
+                compositor = welcome.compositor.as_str(),
+                "host resolved compositor"
+            );
+        }
 
         // Reserve our data-plane port, then start the host.
         let probe = std::net::UdpSocket::bind("0.0.0.0:0")?;