feat(qbz-qt): phase 14 — app-wide ambient background (modo Cider)

Port of the spec's palette derivation (16x16 downscale, coverage-
weighted 24-bin hue histogram, genuine secondary cluster or deeper
same-hue, +180 accent, B/W defaults) driven by the now-playing artwork
on track change. QPainter AmbientField with the WGSL scene's exact
four blob orbits, dim scrim, 30fps cap paused when inactive; chrome
surfaces translucent per the AppShell alpha tokens (content frame
surface-main @ 0.22 frosted). Same shared app_background pref (owner
has it 'ambient'), app-menu toggle applying live, QBZ_BG_* env knobs.
4 new unit tests (14/14 total).

km

crates/qbz-qt/build.rs

@@ -57,6 +57,7 @@ fn main() {
                 "qml/QbzScrollBar.qml",
                 "qml/QbzSpinner.qml",
                 "qml/RoundedImage.qml",
+                "qml/AmbientField.qml",
                 "qml/QbzContextMenu.qml",
             ],
             qrc_files: &qrc_refs,