Replace placeholder example in replay-attacks.md

`chapter-03-crypto-cryptography/09_replay-attacks.md` line 140 has a stand-in comment:

```
// This is a placeholder for demonstration
```

Replace with a real, runnable replay-attack demonstration (per the book's "code complete" goal — samples should run by copy/paste).

Tracked in TODO.md.

Open chapter-03-crypto-cryptography/09_replay-attacks.md around line 140 and review TODO.md for the tracked context. Study the surrounding example and the book's runnable-sample conventions, then replace the placeholder with a complete demonstration that runs by copy/paste; verify the sample executes successfully.

Written by the indexing model from the issue text.