perf(mornye): reuse exact luminance contributions for contrast sampling

Use 6 KiB of lookup tables to avoid repeated Color allocations and exponentiation during frame sampling. Exhaustively verify exact Flutter luminance parity for every 8-bit RGB value.

lib/widgets/mornye_artwork_contrast.dart

@@ -1,9 +1,35 @@
 import 'dart:async';
+import 'dart:typed_data';
 import 'dart:ui' as ui;
 
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 
+// Readback uses 8-bit sRGB. Cache each channel's exact Flutter luminance
+// contribution (6 KiB total), avoiding three powers and a Color per pixel.
+final _redLuminance = Float64List.fromList(
+  List.generate(
+    256,
+    (value) => Color.fromARGB(255, value, 0, 0).computeLuminance(),
+  ),
+);
+final _greenLuminance = Float64List.fromList(
+  List.generate(
+    256,
+    (value) => Color.fromARGB(255, 0, value, 0).computeLuminance(),
+  ),
+);
+final _blueLuminance = Float64List.fromList(
+  List.generate(
+    256,
+    (value) => Color.fromARGB(255, 0, 0, value).computeLuminance(),
+  ),
+);
+
+/// Matches [Color.computeLuminance] for an RGB pixel from an 8-bit readback.
+double artworkPixelLuminance(int red, int green, int blue) =>
+    _redLuminance[red] + _greenLuminance[green] + _blueLuminance[blue];
+
 /// Samples rendered video and its fade, rather than a static album palette.
 /// Read back only 48 pixels across, at most three times a second when visible.
 class MornyeArtworkContrast extends StatefulWidget {
@@ -95,12 +121,11 @@ class _MornyeArtworkContrastState extends State<MornyeArtworkContrast> {
         for (var y = top; y < bottom; y++) {
           for (var x = left; x < right; x++) {
             final index = (y * image.width + x) * 4;
-            luminance += Color.fromARGB(
-              255,
+            luminance += artworkPixelLuminance(
               pixels.getUint8(index),
               pixels.getUint8(index + 1),
               pixels.getUint8(index + 2),
-            ).computeLuminance();
+            );
             count++;
           }
         }

test/mornye_artwork_contrast_test.dart

@@ -3,6 +3,25 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:spotiflac_android/widgets/mornye_artwork_contrast.dart';
 
 void main() {
+  test('readback luminance matches Flutter exactly for every RGB pixel', () {
+    for (var red = 0; red < 256; red++) {
+      for (var green = 0; green < 256; green++) {
+        for (var blue = 0; blue < 256; blue++) {
+          final expected = Color.fromARGB(
+            255,
+            red,
+            green,
+            blue,
+          ).computeLuminance();
+          final actual = artworkPixelLuminance(red, green, blue);
+          if (actual != expected) {
+            fail('RGB($red, $green, $blue): $actual != $expected');
+          }
+        }
+      }
+    }
+  });
+
   testWidgets('foreground follows changing frames separately for each region', (
     tester,
   ) async {