fix: 修复非 UTF-8 编码文件读写 round-trip 字节损坏

GBK 文件编辑后被错误写入为 UTF-8（Buffer.from 不支持 gbk 编码，
encodeString 静默 fallback），latin1/ANSI 文件 0x80-0x9F 范围字节因
TextDecoder('latin1') 与 Buffer.from('latin1') 编解码不对称而被篡改。

修复：latin1 解码改用严格 ISO-8859-1 映射保证与 Buffer.from 对称；
GBK 编码通过 TextDecoder 反向构建查找表实现零依赖编码器。

Co-Authored-By: glm-5-turbo <zai-org@claude-code-best.win>

src/utils/__tests__/encoding.test.ts

@@ -70,6 +70,17 @@ describe('decodeBuffer', () => {
     const buf = Buffer.alloc(0)
     expect(decodeBuffer(buf, 'utf-8')).toBe('')
   })
+
+  test('decodes latin1 using strict ISO-8859-1 mapping', () => {
+    // 0x80 should decode to U+0080 (control char), NOT € (U+20AC)
+    const buf = Buffer.from([0x80, 0x85, 0x9c, 0xa0, 0xff])
+    const decoded = decodeBuffer(buf, 'latin1')
+    expect(decoded.charCodeAt(0)).toBe(0x80)
+    expect(decoded.charCodeAt(1)).toBe(0x85)
+    expect(decoded.charCodeAt(2)).toBe(0x9c)
+    expect(decoded.charCodeAt(3)).toBe(0xa0)
+    expect(decoded.charCodeAt(4)).toBe(0xff)
+  })
 })
 
 describe('encodeString', () => {
@@ -91,12 +102,71 @@ describe('encodeString', () => {
     expect(decodeBuffer(buffer, 'utf-16le')).toBe('Hello')
   })
 
-  test('handles GBK encoding (may convert)', () => {
+  test('encodes GBK string correctly', () => {
     const { buffer, converted } = encodeString('你好', 'gbk')
-    expect(buffer).toBeInstanceOf(Buffer)
-    expect(typeof converted).toBe('boolean')
-    if (!converted) {
-      expect(decodeBuffer(buffer, 'gbk')).toBe('你好')
-    }
+    expect(converted).toBe(false)
+    expect(buffer.toString('hex')).toBe('c4e3bac3')
+  })
+
+  test('GBK round-trip preserves bytes', () => {
+    // "测试文件" in GBK
+    const original = Buffer.from([
+      0xb2, 0xe2, 0xca, 0xd4, 0xce, 0xc4, 0xbc, 0xfe,
+    ])
+    const decoded = decodeBuffer(original, 'gbk')
+    const { buffer } = encodeString(decoded, 'gbk')
+    expect(buffer.equals(original)).toBe(true)
+  })
+
+  test('GBK encoding handles mixed ASCII and CJK', () => {
+    // "Hello你好" in GBK: 48 65 6c 6c 6f c4 e3 ba c3
+    const { buffer, converted } = encodeString('Hello你好', 'gbk')
+    expect(converted).toBe(false)
+    expect(buffer.toString('hex')).toBe('48656c6c6fc4e3bac3')
+  })
+
+  test('latin1 round-trip preserves all byte values', () => {
+    // Test the full 0x80-0xFF range that previously broke
+    const bytes = Buffer.from([
+      0x80, 0x81, 0x85, 0x8c, 0x9c, 0xa0, 0xc0, 0xe9, 0xf6, 0xfc, 0xff,
+    ])
+    const decoded = decodeBuffer(bytes, 'latin1')
+    const { buffer } = encodeString(decoded, 'latin1')
+    expect(buffer.equals(bytes)).toBe(true)
+  })
+
+  test('latin1 encoding does not set converted flag', () => {
+    const { buffer, converted } = encodeString('test\x80\x90', 'latin1')
+    expect(converted).toBe(false)
+    expect(buffer.toString('hex')).toBe('746573748090')
+  })
+})
+
+describe('round-trip consistency', () => {
+  test('GBK file survives full read-decode-encode cycle', () => {
+    const original = Buffer.from([0xc4, 0xe3, 0xba, 0xc3, 0x0d, 0x0a])
+    const enc = detectEncoding(original)
+    expect(enc).toBe('gbk')
+    const decoded = decodeBuffer(original, enc)
+    const { buffer } = encodeString(decoded, enc)
+    expect(buffer.equals(original)).toBe(true)
+  })
+
+  test('latin1 file survives full read-decode-encode cycle', () => {
+    const original = Buffer.from([0x80, 0x90, 0xa0, 0xff, 0x41, 0x42])
+    const enc = detectEncoding(original)
+    expect(enc).toBe('latin1')
+    const decoded = decodeBuffer(original, enc)
+    const { buffer } = encodeString(decoded, enc)
+    expect(buffer.equals(original)).toBe(true)
+  })
+
+  test('UTF-8 file survives full read-decode-encode cycle', () => {
+    const original = Buffer.from('Hello 世界', 'utf-8')
+    const enc = detectEncoding(original)
+    expect(enc).toBe('utf-8')
+    const decoded = decodeBuffer(original, enc)
+    const { buffer } = encodeString(decoded, enc)
+    expect(buffer.equals(original)).toBe(true)
   })
 })