fix: derive Windows image keys from local metadata

tests/unit/wechat-image-key-derivation.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from 'vitest'
+import {
+  deriveV4ImageKeys,
+  extractKvcommCode,
+  normalizeV4AccountId
+} from '../../src/shared/wechat-image-key-derivation'
+
+describe('WeChat 4 image key derivation', () => {
+  it('normalizes account directories without leaking the per-install suffix', () => {
+    expect(normalizeV4AccountId('C:\\data\\wxid_fixture_ab12')).toBe('wxid_fixture')
+    expect(normalizeV4AccountId('/data/custom-account_ab12')).toBe('custom-account')
+  })
+
+  it('extracts only valid uint32 kvcomm codes', () => {
+    expect(extractKvcommCode('key_123456789_network.statistic')).toBe(123456789)
+    expect(extractKvcommCode('key_0_network.statistic')).toBeNull()
+    expect(extractKvcommCode('other_123_network.statistic')).toBeNull()
+  })
+
+  it('derives the documented XOR byte and 16-character AES key', () => {
+    expect(deriveV4ImageKeys(123456789, 'wxid_fixture_ab12')).toEqual({
+      xorKey: 0x15,
+      aesKey: '3bc2c5cbb10dfeda'
+    })
+  })
+})