node: add turnkey BroadcastNode Host wrapper

[ch4r10t33r]

What

Adds src/node/broadcast_host.zig — a turnkey, poll-driven BroadcastNode Host that packages the QUIC RS-broadcast assembly which until now existed only as test sequences in transport/eth_ec_quic_enabled.zig.

A raw EngineQuicHost bridges one inbound peer's SESS/CHUNK streams into an Engine; everything else (bind, dial, the BCAST handshake, the peer map, the poll loop, the outbound send, and the decode-and-deliver trigger) was left to the caller. BroadcastNode fills those gaps:

• bind + connect + BCAST handshake — owns a QUIC listen endpoint (server) and a dial endpoint (client); connect/acceptInbound complete the QUIC handshake then run the BCAST handshake exchange and capture the remote peer id.
• peer map + tick(now_ms) — single-threaded poll loop: pump QUIC, accept inbound, advance handshakes, decode+deliver, GC. No goroutines/tickers, matching the library's synchronous port style.
• publish — RS-encodes the payload, sends the SESS session_open (preamble) then the RS shard chunks to each ready peer.
• decode-and-deliver — polls progress(), calls sessionDecode when a relay session has enough shards, and hands the FullMessage to the channel's Subscription. Origin sessions are skipped (no self-delivery).

Test

Loopback self-interop test (registered under the test-quic CI job): node A publishes a message and node B reconstructs the identical payload over real QUIC. Verified locally with the 0.16.0 toolchain (test-quic 59/59 green; default zig build clean; zig fmt --check clean).

Scope / limitations

Deliberately experimental, mirroring the parallel-broadcast adapter pattern:

• The outbound path is single-peer-correct — the RS emit planner tags each dispatch with a target peer, but SendRsChunkFn is peer-agnostic, so one sessionDrainOutboundOverQuic call fans one session to one connection.
• No multi-peer shard routing, peer authentication, or peer-disconnect events.
• The co_poll_ep hook exists for in-process loopback tests; real deployments pass null (each process pumps its own socket via tick).