fix: eliminate AVIF/WebP grey halo on white backgrounds (v0.2.3)

Diagnosed visible faint grey rectangle around B&W ink art images on
pages with pure-white backgrounds. Root cause was format encoder
luminance shift in near-white pixels:

  Source JPG corner: (253,253,253) = #FDFDFD = 99.2% white
  AVIF q=65 (default): (250,250,250) = #FAFAFA = 98.0% (1.2% halo)
  WebP q=80 (default): (249,249,249) = #F9F9F9 = 97.6% (1.6% halo)

Two changes:

1. avifenc now uses -y 444 (full chroma subsampling) instead of
   default 4:2:0. Brings AVIF corner to #FBFBFB = 98.4%, smaller
   file size as a bonus (~10% reduction on a typical art image).

2. WebP default quality raised 80 → 90. Reaches #FDFDFD = exact
   match with source JPG. File size increases ~30% but eliminates
   the halo entirely for WebP-capable browsers (vast majority).

AVIF still has 0.4% residual halo (libavif 0.11.1 ceiling at this
quality range — pushing higher yields no improvement, only file
size). Acceptable tradeoff: WebP is the served-by-default fallback
when AVIF isn't perfect.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/class-settings.php

@@ -33,7 +33,7 @@ final class Settings {
 			// Format generation
 			'generate_webp'            => true,
 			'generate_avif'            => true,     // design decision 1: on by default everywhere
-			'webp_quality'             => 80,
+			'webp_quality'             => 90,    // matches source JPG luminance for B&W ink on white
 			'avif_quality'             => 65,       // ≈ JPEG q=85 visually
 			'avif_speed'               => 6,        // 0-10; 6 is balanced
 			'avif_async'               => true,     // background WP-Cron so upload UI is responsive