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/cards/LabelCard.qml

@@ -92,7 +92,11 @@ Rectangle {
                 width: 190
                 height: 190
                 radius: 95
-                clip: true
+                // No clip. It never clipped to the circle — QML's clip is a
+                // rectangular scissor and ignores `radius`; the round shape is
+                // each child's own `radius: 95` / RoundedImage mask. The logo
+                // is inset 28px with fit "contain" and the overlay row ends at
+                // y=157 < 190, so nothing overflows.
                 // Logo arm background (the Slint Theme.surface-elevated).
                 color: theme.surfaceElevated