fix: bound native frame parsing before allocation

[RobLe3]

Summary

• enforce the payload-only 16 MiB frame limit before allocation and before encoding
• reject unsupported header and negotiated versions
• require INIT before application frames and reject duplicate INIT
• make network-facing response encoding fallible rather than panic on handler output
• consume the updated portable negative vectors

Local verification

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-features

Boundary

This hardens the optional development TCP binding. It does not claim a production TLS boundary, resolve the relay opcode collision, qualify native TCP for stable use, or implement QUIC.

Refs #158
Refs #159