feat(vdisplay): enumerate the host's physical monitors

P0 of per-monitor capture (design/per-monitor-portal-capture.md): nothing
in the tree could answer "what heads does this host have?", which both a
monitor pin and a console picker need first.

One read per compositor, each beside the code that already speaks that
dialect: KWin's kde_output_device_v2 (the topology session now also records
geometry + scale), Mutter's DisplayConfig.GetCurrentState, swaymsg
get_outputs, hyprctl monitors. Logical geometry throughout, because x/y is
what identifies a head — two monitors can share a size but never an origin.

PUNKTFUNK_CAPTURE_MONITOR parses here and is reported at startup, loudly
including that it is not yet enforced: a knob that is read but inert has to
say so, or "it didn't work" reads as a bug. GET /display/monitors always
answers 200 with an explained empty list, so a compositor-less host renders
as "nothing to pick", not as a broken console.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/mgmt/tests.rs

@@ -1056,6 +1056,29 @@ async fn display_state_and_release_empty() {
     assert_eq!(body["released"], 0);
 }
 
+/// `/display/monitors` is wired, auth-gated, and — the point of the test — **always answers 200
+/// with a well-formed envelope**, including on a test host with no compositor to enumerate. The
+/// console renders a picker from this; an enumeration failure has to arrive as an `error` string
+/// next to an empty list, never as a 5xx that reads to the UI as "the host is broken".
+#[tokio::test]
+async fn display_monitors_answers_even_with_no_compositor() {
+    let app = test_app(test_state(), None);
+
+    let (status, body) = send(&app, get_req("/api/v1/display/monitors")).await;
+    assert_eq!(status, StatusCode::OK);
+    assert!(body["monitors"].is_array(), "monitors is always an array");
+    // No compositor on the test host ⇒ either a clean empty list or an explained failure, never
+    // both empty AND silent about why.
+    let listed = body["monitors"].as_array().map(|a| a.len()).unwrap_or(0);
+    assert!(
+        listed > 0 || !body["error"].is_null() || body["compositor"].is_null(),
+        "an empty list must carry an error or an absent compositor: {body}"
+    );
+    // The pin is reported verbatim so the console can flag "pinned to a monitor you don't have";
+    // unset on the test host.
+    assert!(body["pinned"].is_null(), "no PUNKTFUNK_CAPTURE_MONITOR set");
+}
+
 #[tokio::test]
 async fn native_pairing_arm_show_and_unpair() {
     let np = Arc::new(