fix: bound native frame parsing before allocation

[RobLe3]

Summary

• enforce the payload-only 16 MiB frame limit before allocation and encoding
• reject unsupported header and negotiated versions
• require a valid INIT before application frames and reject duplicate INIT
• apply the same bounded frame checks to the experimental relay path
• consume the updated portable negative vectors

Local verification

• .venv/bin/ruff check src tests scripts
• PATH="$PWD/.venv/bin:$PATH" .venv/bin/python scripts/check_mypy_baseline.py
• .venv/bin/pytest -q (1,230 passed before one stale no-handshake test was corrected)
• .venv/bin/pytest -q tests/test_serve_multiplex.py (4 passed after correction)

Boundary

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