experiment(render): revert sat/gamma/blur to baseline; blue-channel ×0.95

Experiment #3 (sat 115, gamma 1.2, blur 0.6) was a net loss — greens
desaturated, no help on sky→face blue bleed.

Reverting those three to baseline (130, 1.0, 0.0). New experiment #4:
multiply the source's blue channel by 0.95 before dither. Real sky
stays well above the BLUE-vs-WHITE boundary, but borderline-bluish
skin (sky cast on outdoor faces) drops below it and maps to YELLOW /
WHITE / RED instead of feeding the dither's BLUE attractor.

Adds public BLUE_CHANNEL_MUL constant; render-compare's baseline
struct gets blue_mul=1.0 so half-A is still frozen at the original.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Command/RenderCompareCommand.php

@@ -58,6 +58,7 @@ final class RenderCompareCommand extends Command
         'gamma'      => 1.0,
         'sharpen'    => 0.8,
         'blur'       => 0.0,
+        'blue_mul'   => 1.0,
         'dither'     => \Imagick::DITHERMETHOD_FLOYDSTEINBERG,
     ];
 
@@ -94,6 +95,7 @@ final class RenderCompareCommand extends Command
             'gamma'      => RenderImageMessageHandler::GAMMA,
             'sharpen'    => RenderImageMessageHandler::SHARPEN_SIGMA,
             'blur'       => RenderImageMessageHandler::BLUR_SIGMA,
+            'blue_mul'   => RenderImageMessageHandler::BLUE_CHANNEL_MUL,
             'dither'     => RenderImageMessageHandler::DITHER_METHOD,
         ];
 
@@ -140,8 +142,8 @@ final class RenderCompareCommand extends Command
             ? 'FloydSteinberg'
             : ($p['dither'] === \Imagick::DITHERMETHOD_RIEMERSMA ? 'Riemersma' : 'No');
         return sprintf(
-            'sat=%d gamma=%.2f sharpen=%.2f blur=%.2f dither=%s',
-            $p['saturation'], $p['gamma'], $p['sharpen'], $p['blur'], $dither,
+            'sat=%d gamma=%.2f sharpen=%.2f blur=%.2f blue_mul=%.2f dither=%s',
+            $p['saturation'], $p['gamma'], $p['sharpen'], $p['blur'], $p['blue_mul'], $dither,
         );
     }
 
@@ -164,6 +166,9 @@ final class RenderCompareCommand extends Command
         if ($params['blur'] > 0) {
             $im->blurImage(0, $params['blur']);
         }
+        if ($params['blue_mul'] !== 1.0) {
+            $im->evaluateImage(\Imagick::EVALUATE_MULTIPLY, $params['blue_mul'], \Imagick::CHANNEL_BLUE);
+        }
 
         if ($im->getImageWidth() !== self::W || $im->getImageHeight() !== self::FULL_H) {
             $canvas = new \Imagick();