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/messages/de.json

@@ -31,6 +31,11 @@
   "host_codecs": "Codecs",
   "host_ports": "Ports",
   "host_uniqueid": "Eindeutige ID",
+  "host_compositors": "Compositoren",
+  "host_compositors_help": "Backends, auf denen der Host eine virtuelle Ausgabe erzeugen kann. Übergib eine ID an das --compositor-Flag eines Clients; der Host nutzt sie, falls verfügbar, sonst per Auto-Erkennung.",
+  "compositor_available": "Verfügbar",
+  "compositor_unavailable": "Nicht verfügbar",
+  "compositor_default": "Standard",
   "clients_title": "Gekoppelte Geräte",
   "clients_empty": "Noch keine gekoppelten Geräte.",
   "clients_name": "Name",

web/messages/en.json

@@ -31,6 +31,11 @@
   "host_codecs": "Codecs",
   "host_ports": "Ports",
   "host_uniqueid": "Unique ID",
+  "host_compositors": "Compositors",
+  "host_compositors_help": "Backends the host can drive a virtual output on. Pass an id to a client's --compositor flag; the host honors it if available, else auto-detects.",
+  "compositor_available": "Available",
+  "compositor_unavailable": "Unavailable",
+  "compositor_default": "Default",
   "clients_title": "Paired clients",
   "clients_empty": "No paired clients yet.",
   "clients_name": "Name",