feat(qt): ambient background port, clip sweep and measurement tooling

Accumulated qt-frontend work from the 2026-08-11/12 sessions (cc),
uncommitted until now: the app-wide dynamic background in both modes
(AmbientField with GPU/Canvas arms, AppShell layering, ambient palette
pipeline), the wave-1 clip/batch sweep across the tree (alpha batches
449 -> 327, reproducible), the rounded-cover and icon-tint arms, and
scripts/qt-batches.sh (batches + presents/s sampler).

The GPU behaviour of these pieces was finalized in the follow-up pulse
commits; see qbz-nix-docs/qt-frontend/2026-08-11-scenegraph-batches/
GPU-COST-INVESTIGATION.md for the full campaign.

km

crates/qbz-qt/qml/views/LocalAlbumView.qml

@@ -30,7 +30,7 @@ Rectangle {
     id: root
 
     color: ambientOn ? "transparent" : theme.surfaceMain
-    readonly property bool ambientOn: QbzShell.ambientMode > 0 && QbzPlayer.npHasTrack
+    readonly property bool ambientOn: theme.ambientOn
     radius: 12
 
     QbzTheme { id: theme }