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/QbzSelect.qml

@@ -36,7 +36,12 @@ Rectangle {
     radius: sm ? 6 : theme.radiusSm
     border.width: sm ? 0 : 1
     border.color: theme.borderSubtle
-    color: selArea.containsMouse && enabled ? theme.surfaceHover : theme.surfaceElevated
+    // Resting fill goes translucent under the dynamic background so the field
+    // shows through the control (QbzSelect.slint:110-116). Hover keeps its own
+    // token, which is already translucent.
+    color: selArea.containsMouse && enabled
+        ? theme.surfaceHover
+        : (theme.ambientOn ? theme.surfaceElevatedA50 : theme.surfaceElevated)
     opacity: enabled ? 1.0 : 0.4
 
     // `width`, NOT `menuWidth`: QbzSelect.slint:89-91 resolves the open list as