experiment(render): Riemersma dither to stop sky→face blue bleed

Floyd-Steinberg's row-order error diffusion was flushing residual blue
debt from sky pixels downward into the face below — visible as a blue
hue in skin tones that should have been YELLOW/RED/WHITE in the 6-color
palette. Riemersma uses a Hilbert-curve scan, so error stays local and
isn't biased along any axis.

DITHER_METHOD  FLOYDSTEINBERG → RIEMERSMA  (gamma 1.2 + saturation 115
from experiment #1 unchanged).

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

src/MessageHandler/RenderImageMessageHandler.php

@@ -46,8 +46,14 @@ final class RenderImageMessageHandler
     /** sharpenImage(radius=0, SHARPEN_SIGMA). Baseline: 0.8 (light). */
     private const SHARPEN_SIGMA = 0.8;
 
-    /** Imagick dither method. Baseline: DITHERMETHOD_FLOYDSTEINBERG. */
-    private const DITHER_METHOD = \Imagick::DITHERMETHOD_FLOYDSTEINBERG;
+    /** Imagick dither method.
+     *  Baseline:     DITHERMETHOD_FLOYDSTEINBERG — error diffuses down-right
+     *                in row order, so a blue sky's residual error
+     *                accumulates downward and "flushes" into faces/hair
+     *                below, contaminating skin tones with blue.
+     *  Experiment #2: DITHERMETHOD_RIEMERSMA — Hilbert-curve scan, error
+     *                stays local and doesn't bias along any axis. */
+    private const DITHER_METHOD = \Imagick::DITHERMETHOD_RIEMERSMA;
 
     public function __construct(
         private readonly ImageRepository        $imageRepo,