Cache tokenizer reverse byte map (bytesToString perf)

[m96-chan]

tokenizer.ts:382-399 rebuilds the unicode-to-byte reverse map from scratch on every call to bytesToString():

private bytesToString(token: string): string {
    const b2u = Tokenizer.getByteToUnicode();
    const u2b = new Map<string, number>();   // rebuilt every call (256 entries)
    for (const [k, v] of b2u) {
      u2b.set(v, k);
    }
    // ...
}

This is called once per token during streaming generation — a hot path. Caching the reverse map as a static field eliminates 256 Map.set operations per decoded token.