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>

docs/api/openapi.json

@@ -103,6 +103,41 @@
         }
       }
     },
+    "/api/v1/compositors": {
+      "get": {
+        "tags": [
+          "host"
+        ],
+        "summary": "Available compositor backends",
+        "description": "Lists every backend the host knows how to drive, flags which are usable right now, and marks\nthe one an unspecified (`Auto`) client request resolves to. Clients pass an `id` to their\n`--compositor` flag (or `PUNKTFUNK_COMPOSITOR_*` over the C ABI) to request it.",
+        "operationId": "listCompositors",
+        "responses": {
+          "200": {
+            "description": "Compositor backends with availability + the auto-detected default",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "array",
+                  "items": {
+                    "$ref": "#/components/schemas/AvailableCompositor"
+                  }
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "Missing or invalid bearer token",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ApiError"
+                }
+              }
+            }
+          }
+        }
+      }
+    },
     "/api/v1/health": {
       "get": {
         "tags": [
@@ -381,6 +416,34 @@
           }
         }
       },
+      "AvailableCompositor": {
+        "type": "object",
+        "description": "A compositor backend the host can drive a virtual output on, and whether it's usable now.",
+        "required": [
+          "id",
+          "label",
+          "available",
+          "default"
+        ],
+        "properties": {
+          "available": {
+            "type": "boolean",
+            "description": "Usable on this host right now: the live session's own compositor, or gamescope wherever\nits binary is installed."
+          },
+          "default": {
+            "type": "boolean",
+            "description": "True for the backend an `Auto` (unspecified) request resolves to right now."
+          },
+          "id": {
+            "type": "string",
+            "description": "Stable identifier (`\"kwin\"` | `\"wlroots\"` | `\"mutter\"` | `\"gamescope\"`) — pass this to a\nclient's `--compositor` flag."
+          },
+          "label": {
+            "type": "string",
+            "description": "Human-readable label for UIs."
+          }
+        }
+      },
       "Health": {
         "type": "object",
         "description": "Liveness + version probe.",