fix: 解码器3个bug修复 — 编码→解码往返恢复 + 新增往返测试

1. detect.rs: remove()索引bug — 元素前移后索引未更新，改用每次remove(0)
2. detect.rs: 版本估算公式修正 — (dist-14)/4 → (dist-10)/4，符合ZXing公式
3. extract.rs: 移除显式col 6跳过 — read_module已自动跳过保留区，显式skip导致列配对错位/行序反转

新增 test_roundtrip_png: 矩阵往返 + PNG往返双验证

core/src/decoder/detect.rs

@@ -145,7 +145,7 @@ pub(crate) fn estimate_version_from_tl_tr(tl: (f64, f64), tr: (f64, f64), module
     let dy = tr.1 - tl.1;
     let dist_px = (dx * dx + dy * dy).sqrt() as f32;
     let dist_modules = dist_px / module_size as f32;
-    let ver = ((dist_modules as i32 - 14) / 4) as u8;
+    let ver = ((dist_modules as i32 - 10) / 4) as u8;
     ver.clamp(1, 40)
 }
 
@@ -226,9 +226,10 @@ fn find_finders(gray: &[Vec<bool>]) -> Option<[FinderMatch; 3]> {
         finders.swap(1, 2);
     }
 
+    // remove 会使后续元素前移，必须每次 remove(0)
     let f0 = finders.remove(0);
-    let f1 = finders.remove(1);
-    let f2 = finders.remove(2);
+    let f1 = finders.remove(0);
+    let f2 = finders.remove(0);
 
     // ZXing 几何验证：module_size 一致性 + 勾股定理
     if !validate_finder_geometry(&[&f0, &f1, &f2]) {
@@ -303,7 +304,9 @@ pub(crate) fn detect_and_extract(
     let dy = tr.cy as f64 - tl.cy as f64;
     let dist_px = (dx * dx + dy * dy).sqrt() as f32;
     let dist_modules = dist_px / module_size as f32;
-    let ver = ((dist_modules as i32 - 14) / 4) as u8;
+    // ZXing 公式: totalModules = dist/moduleSize + 7, version = (totalModules - 17) / 4
+    // 简化为: version = (dist_modules + 7 - 17) / 4 = (dist_modules - 10) / 4
+    let ver = ((dist_modules as i32 - 10) / 4) as u8;
     let version = ver.clamp(1, 40);
 
     let size = 17 + version as usize * 4;

core/src/decoder/extract.rs

@@ -32,11 +32,8 @@ fn extract_bits(matrix: &Matrix, total_codewords: usize) -> Vec<bool> {
         }
         col -= 2;
         going_up = !going_up;
-
-        // 跳过垂直时序图案列（col 6）
-        if col == 6 {
-            col -= 1;
-        }
+        // 垂直时序图案列（col 6）由 read_module 自动跳过保留区，
+        // 无需显式 skip，否则会导致列配对错位、行序反转
     }
 
     bits.truncate(target_bits);

core/src/qr.rs

@@ -419,4 +419,26 @@ mod tests {
         assert!(svg.contains("#FF0000"));
         assert!(svg.contains("#0000FF"));
     }
+
+    #[test]
+    fn test_roundtrip_png() {
+        let config = QrConfig {
+            level: EcLevel::M,
+            version: VersionMode::Auto,
+            margin: 4,
+        };
+        let qr = QrCode::encode("Hello World", config).unwrap();
+
+        // 直接从矩阵解码
+        let matrix: Vec<Vec<bool>> = qr.modules().to_vec();
+        let result = crate::decoder::decode_matrix(&matrix)
+            .expect("矩阵解码往返失败");
+        assert_eq!(result.text, "Hello World");
+
+        // PNG 往返
+        let png_bytes = qr.to_png_bytes(8, None).unwrap();
+        let result = crate::decoder::decode_image(&png_bytes)
+            .expect("PNG 解码往返失败");
+        assert_eq!(result.text, "Hello World");
+    }
 }