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/AlbumListRow.qml

@@ -152,7 +152,9 @@ Rectangle {
                 anchors.centerIn: parent
                 radius: 4
                 color: theme.surfaceElevated
-                clip: true
+                // No clip: RoundedImage confines its own crop on both arms.
+                // One batch root per list row, for a scissor that never
+                // rounded anything.
                 RoundedImage {
                     anchors.fill: parent
                     source: root.artSource !== "" ? root.artSource : (root.item.artPath || "")