feat(lyrics): lite-fill display pref — static active-line highlight to cut CPU

The per-word karaoke sweep animates a clip rect ~60x/s, driving continuous full-window repaints (~4% CPU). Lite fill highlights the active line whole (no per-frame animation) so the surface only repaints on line changes. New toggle in the lyrics controls flyout, persisted via lyrics_prefs; default off (smooth karaoke). Kiosk / weak-GPU tiers force it on via ShellState.reduce-motion.

cc

crates/qbz-ui/ui/shell/LyricsControlsFlyout.slint

@@ -176,6 +176,21 @@ export component LyricsControlsPanel inherits Rectangle {
             }
         }
 
+        // Lite fill (perf): highlight the active line whole instead of the
+        // per-word karaoke sweep — stops the continuous full-window repaints.
+        HorizontalLayout {
+            height: 22px;
+            spacing: 8px;
+            RowLabel { text: @tr("Lite fill (saves CPU)"); }
+            QbzToggle {
+                checked: LyricsState.lite-fill;
+                toggled(v) => {
+                    LyricsState.lite-fill = v;
+                    LyricsState.prefs-changed();
+                }
+            }
+        }
+
         // Dimming (Tauri segmented Off/Soft/Strong -> select).
         HorizontalLayout {
             height: 30px;

crates/qbz-ui/ui/shell/LyricsLinesView.slint

@@ -44,7 +44,7 @@
 
 import { Theme } from "../foundation/semantic-colors.slint";
 import { Typography } from "../foundation/typography.slint";
-import { LyricsState, LyricsSegment } from "../state.slint";
+import { LyricsState, LyricsSegment, ShellState } from "../state.slint";
 
 // Q6 (SHIPPED): Tauri's four non-system lyric font families, embedded into
 // the binary at compile time (slint-build default EmbedAllResources for Rust
@@ -218,17 +218,24 @@ component LyricsLineRow inherits VerticalLayout {
                 // Bright (sung) overlay, clipped to this segment's LOCAL fill.
                 // local = clamp((line-progress - start-weight) / width-weight, 0, 1)
                 Rectangle {
+                    // Lite fill (pref) or reduce-motion (kiosk / weak GPU): the
+                    // active line is fully lit and STATIC — no per-frame clip
+                    // animation, so the surface stops repainting the whole window
+                    // ~60x/s just to sweep the karaoke fill.
+                    property <bool> cheap: ShellState.reduce-motion || LyricsState.lite-fill;
                     x: 0;
                     y: 0;
-                    width: segbase.width * Math.clamp(
-                        seg.width-weight > 0
-                            ? (LyricsState.line-progress - seg.start-weight) / seg.width-weight
-                            : 0,
-                        0.0, 1.0);
+                    width: self.cheap
+                        ? segbase.width
+                        : segbase.width * Math.clamp(
+                            seg.width-weight > 0
+                                ? (LyricsState.line-progress - seg.start-weight) / seg.width-weight
+                                : 0,
+                            0.0, 1.0);
                     height: segbase.height;
                     clip: true;
                     animate width {
-                        duration: LyricsState.fill-anim-ms * 1ms;
+                        duration: self.cheap ? 0ms : LyricsState.fill-anim-ms * 1ms;
                         easing: linear;
                     }
                     Text {

crates/qbz-ui/ui/state.slint

@@ -4220,6 +4220,12 @@ export global LyricsState {
     // 0 off · 1 soft · 2 strong (Tauri default 'strong').
     in-out property <int> dimming-mode: 2;
     in-out property <bool> uppercase: false;
+    // Lite fill (perf): when true, the active line is highlighted whole instead
+    // of the per-word karaoke sweep — no per-frame clip animation, so the lyrics
+    // surface stops driving continuous full-window repaints (~4% CPU -> idle
+    // between line changes). Default false (smooth karaoke). Kiosk / weak-GPU
+    // tiers force it on via ShellState.reduce-motion regardless of this pref.
+    in-out property <bool> lite-fill: false;
     // Active (karaoke fill) color. `use-custom-color: false` = follow the theme
     // accent (Tauri activeColor: ''). The palette swatches set both (deviation
     // D6: swatches instead of a native color input — Slint has none).