fix(qt): bound the queue panels, and make the gate catch what it missed

Follow-up to 66ba12bd5, which fixed the coverflow but left two of its
neighbours and a gate that could not see either of them.

QueueTabsPanel built one ImmQueueRow per queue track when opened —
including every collapsed 0-height history slot, which on the reported
session was 1162 of 1935. The slot stays (its height is what makes the
collapsed-history arithmetic work); the row inside is now built only
within a viewport of the visible band.

SuggestionsPanel and AppShell's QueuePanel move behind Loaders. The
queue panel was never the freeze — its rows are paginated at 40 in
Rust — but a closed panel should not be constructed, and the audit
cannot know about a cap that lives on the other side of the bridge.
Gating it also let SuggestionsPanel drop the onVisibleChanged half of
its load hook: mounting IS becoming visible now.

The audit is the real content of this commit. Written narrow — only a
model fed by unbounded user data counts, because the wide rule reports
145 sites and a gate that noisy gets switched off in a day — it then
reported the pre-fix tree CLEAN. A gate that cannot catch the bug it
was written for is worth nothing, so it was tested against that tree
and fixed on two counts:

  - CROSS-FILE. The `visible:` sat in ImmersiveView.qml while the two
    Repeaters over the whole queue lived in CoverflowPanel.qml. There
    is now a transitive index: a component is heavy if its own file has
    an unbounded view or if it mounts something that does.
  - MULTI-LINE. The coverflow's gate spans two lines, so a first-line
    read saw `QbzShaderScene.scene === 0`, matched no known gate, and
    skipped it.

With both, it finds all five mounts on the pre-fix tree, including the
two that froze the app. Known gates also grew past tab predicates: the
coverflow was keyed on QbzImmersive.mode.

Verified on a copy of the real session state with the 1935-row queue
restored: launches to a painted Discover, idle at 1.0% CPU.

cc

crates/qbz-qt/qml/rows/TrackRow.qml

@@ -1055,11 +1055,26 @@ Rectangle {
             }
         }
     }
-    CardMenu {
-        id: rowMenu
-        menuWidth: 224
-        entries: root.menuModel()
-        onPicked: function (a) { root.menuAction(a) }
+    // LAZY. This is a QtQuick.Controls Popup with a Repeater over its
+    // entries, and it was built EAGERLY for every row — so a long list
+    // constructed one whole popup per visible row, and rebuilt them all
+    // on every scroll step, to show a menu only ever opened on click.
+    // Same lazy-Loader idiom this file already uses for the clipboard
+    // helper and the info modal.
+    Loader {
+        id: rowMenuLoader
+        active: false
+        sourceComponent: CardMenu {
+            menuWidth: 224
+            entries: root.menuModel()
+            onPicked: function (a) { root.menuAction(a) }
+        }
+    }
+    /// Build the popup on first use, then open it. Called by `openRowMenu`
+    /// below, which owns the empty-menu guard.
+    function openRowMenuLazy(anchor, x, y) {
+        rowMenuLoader.active = true
+        rowMenuLoader.item.openAtCursor(anchor, x, y)
     }
 
     // TrackContextMenu.slint, in its order. Every row here reaches a live
@@ -1074,7 +1089,7 @@ Rectangle {
     function openRowMenu(anchor, x, y) {
         if (root.menuModel().length === 0)
             return
-        rowMenu.openAtCursor(anchor, x, y)
+        root.openRowMenuLazy(anchor, x, y)
     }
 
     function menuModel() {