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/controls/BrowseGenreButton.qml

@@ -41,8 +41,13 @@ Rectangle {
     width: genreRow.width
     height: btnHeight
     radius: 6
+    // Resting fill translucent under the dynamic background
+    // (HomeView.slint:89, BrowseHeaderTools.slint:111, PlaylistTagFilter
+    // .slint:77 — the same chip in three mounts). Accent and hover keep theirs.
     color: root.active ? theme.accent
-         : genreArea.containsMouse ? theme.surfaceHover : theme.surfaceElevated
+         : genreArea.containsMouse
+             ? theme.surfaceHover
+             : (theme.ambientOn ? theme.surfaceElevatedA50 : theme.surfaceElevated)
 
     Row {
         id: genreRow