修复完美像素对小于 4 像素软网格识别偏大

峰间距过滤器 4 会把 2–3 像素网格的相邻峰成对合并成 5–6，再取 P30 就把
683×1024 生成素材统一报成 5–6 像素格；改为 2 后不再合并峰。同时 walker
窗口起点向上取整且不早于上一条切线 +2，避免小步长时切出 1 像素格。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server-rs/crates/platform-image/src/pixel_art_snapper.rs

@@ -28,7 +28,10 @@ const MAX_DECODE_ALLOC_BYTES: u64 = PIXEL_ART_MAX_IMAGE_PIXELS * 16;
 const KMEANS_SEED: u64 = 42;
 const MAX_KMEANS_ITERATIONS: usize = 15;
 const PEAK_THRESHOLD_MULTIPLIER: f64 = 0.2;
-const PEAK_DISTANCE_FILTER: usize = 4;
+// Two peaks can never be adjacent (the left `>=` / right `>` test forbids it), so `2`
+// keeps every peak. The previous `4` merged the 2–3 px gaps of soft sub-4 px grids
+// into 5–6 px ones and made every such grid come out about twice too coarse.
+const PEAK_DISTANCE_FILTER: usize = 2;
 const STEP_SIZE_PERCENTILE: f64 = 0.3;
 const WALKER_SEARCH_WINDOW_RATIO: f64 = 0.35;
 const WALKER_MIN_SEARCH_WINDOW: f64 = 2.0;
@@ -828,8 +831,12 @@ fn walk(
             break;
         }
 
+        // The window start is rounded up and never closer than 2 px to the previous cut:
+        // with steps of 2–3 px the fixed minimum window would otherwise reach back to
+        // `current + 1` and split 1 px cells off the real edge.
         let start = (target - search_window)
-            .max(current_position + 1.0)
+            .ceil()
+            .max(current_position + 2.0)
             .max(0.0) as usize;
         let end = ((target + search_window) as usize).min(limit);
         if end <= start {