TransportCapability enum classifies transports by bandwidth/MTU:
- Unconstrained (≥1 Mbps): Full MLS with PQ-KEM
- Medium (≥10 kbps): Full MLS classical
- Constrained (≥1 kbps): MLS-Lite with signature
- SeverelyConstrained (<1 kbps): MLS-Lite minimal
TransportManager now provides:
- best_transport() - highest capability transport
- recommended_crypto() - appropriate crypto mode
- supports_mls() - whether any transport handles full MLS
- select_for_size() - best transport for a given payload
CryptoMode enum with overhead estimates for each mode.