transport (future): track 001-ethp2p transport features beyond current lsquic stack

## Context

[001-ethp2p.md](https://github.com/ethp2p/ethp2p/blob/main/specs/001-ethp2p.md) **Transport** section targets: TLS **ECH**, **BBRv3** / Prague CC, **TCP + QMux** fallback, multipath QUIC, stream prioritization tied to slot phases, etc. Status in that doc: *Designing*.

Today `zig-ethp2p` uses **lsquic + BoringSSL** with TLS 1.3, ALPN `eth-ec-broadcast`, and UNI streams (`README.md`, `UPSTREAM.md`).

## Goal

This issue is a **roadmap umbrella** — not immediate work. Use it to:

- Record which 001 transport bullets are in scope for this repo vs delegated to zeam/libp2p.
- Link sub-issues when a concrete feature (e.g. congestion-control selection, ECH) becomes implementable with the chosen QUIC stack.

## Note

Full libp2p (Noise, multistream, Yamux) remains out of scope per `README.md`; raw QUIC alignment with ethp2p `sim/host.go` is the current baseline.