feat: 解码器全面增强 — 多遍二值化 + 4点透视 + ECI/FNC1

Phase 1 — 二值化+检测增强:
- decoder/image.rs 重写: 中位数→Otsu→Sauvola 三遍自适应阈值
- 新增 load_gray() 公共 API 供调用方自定义预处理
- 反色QR码自动检测(5点采样法)
- 静区验证 + 包围盒(BoundingBox) + 解码置信度评分
- 定位图案比例偏差计算

Phase 2 — 4点透视变换+多QR码:
- decoder/perspective.rs 重写: DLT求解3x3单应矩阵
- 用3个定位图案+推算第4点构建4点对应
- 双线性插值替换最近邻采样
- 多角度旋转回退(7个预设角度)
- decode_all() 返回图像中所有QR码(按置信度降序)

Phase 3 — ISO合规:
- decoder/bch.rs: decode_format_info/version_info 返回汉明距离
- decoder/format.rs: 副本冲突时选汉明距离更小者
- decoder/mode_decode.rs: ECI指派号识别, FNC1/GS1检测(0101/1001)
- 结构化追加(0011)头部提取: position/total/parity
- DecodeResult 扩展: is_gs1, eci_designator, structured_append

验证: cargo check+clippy通过, 81+24测试全部通过

Cargo.lock

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+ "quote",
+ "syn 2.0.117",
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+ "nalgebra",
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