Use exact AES-GCM bounds and model random ChaCha20-Poly1305 nonces

Refine limit derivations and recommendation wording

Add random-nonce mode, per-key message limit, and deep links

Tighten AEGIS limit derivations

Model 256-bit tag variants as first-class algorithms

Refine AEGIS 256-bit tag claims and random-nonce guidance

Update limit engine and UI

Add AEAD limits computation script