fix: 4 个 CRITICAL bug 修复

- C1: placement.rs 删除列偏移特殊处理 (col==6→5)，place_bit 已自动跳过保留区
- C2: version.rs V5-H 纠错表 h_g1: 4→2 (总码字数 200→134)
- C3: mode.rs Kanji 编码删除冗余 if/else 重复分支
- C4: galois.rs div() 返回 Option<u8> 替代 panic!

core/src/encoder/mode.rs

@@ -158,22 +158,14 @@ fn unicode_to_shift_jis(c: char) -> Option<u16> {
         // 简化映射: 用 Unicode 码位偏移做近似
         // 真实转换需要完整映射表，这里做合理近似
         let base = code - 0x4E00;
-        let hi = 0x81 + (base / 0xBC) as u32;
-        let lo = 0x40 + (base % 0xBC) as u32;
+        let hi = 0x81 + (base / 0xBC);
+        let lo = 0x40 + (base % 0xBC);
         let sjis = ((hi << 8) | lo) as u16;
-        // 映射到 13-bit 码字
-        let val = if sjis <= 0x9FFC {
-            let h = (sjis >> 8) as u16;
-            let l = (sjis & 0xFF) as u16;
-            if h >= 0x81 && h <= 0x9F {
-                (h - 0x81) * 0xBC + (l - 0x40)
-            } else {
-                (h - 0xC1) * 0xBC + (l - 0x40)
-            }
-        } else {
-            let h = (sjis >> 8) as u16;
-            let l = (sjis & 0xFF) as u16;
-            if h >= 0x81 && h <= 0x9F {
+        // 映射到 13-bit 码字（内层 if/else 已区分两个 Shift-JIS 区间）
+        let val = {
+            let h = (sjis >> 8);
+            let l = (sjis & 0xFF);
+            if (0x81..=0x9F).contains(&h) {
                 (h - 0x81) * 0xBC + (l - 0x40)
             } else {
                 (h - 0xC1) * 0xBC + (l - 0x40)