feat: add cch hash computation module using xxHash64

Implements the cch request signing algorithm described in the
reverse-engineering article. Uses xxhash-wasm for xxHash64 computation
with the known seed constant.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/utils/cch.ts

@@ -0,0 +1,28 @@
+import xxhash from 'xxhash-wasm'
+
+const CCH_SEED = 0x6E52736AC806831En
+const CCH_PLACEHOLDER = 'cch=00000'
+const CCH_MASK = 0xFFFFFn
+
+let hasherPromise: ReturnType<typeof xxhash> | null = null
+
+function getHasher() {
+  if (!hasherPromise) {
+    hasherPromise = xxhash()
+  }
+  return hasherPromise
+}
+
+export async function computeCch(body: string): Promise<string> {
+  const hasher = await getHasher()
+  const hash = hasher.h64Raw(new TextEncoder().encode(body), CCH_SEED)
+  return (hash & CCH_MASK).toString(16).padStart(5, '0')
+}
+
+export function replaceCchPlaceholder(body: string, cch: string): string {
+  return body.replace(CCH_PLACEHOLDER, `cch=${cch}`)
+}
+
+export function hasCchPlaceholder(body: string): boolean {
+  return body.includes(CCH_PLACEHOLDER)
+}