Change webrtc lib for udp request

[Tchips46]

Problem

wrtc@0.4.7 was last published 2023-10-12 and has never actually worked in this
repo. require("wrtc") fails with Cannot find module '../build/Release/wrtc.node' —
the native binding was never built. Both server specs vi.mock("wrtc", …), so nothing
ever loaded it.

It also breaks pnpm install for everyone: wrtc ships node-pre-gyp as a
bundledDependency, which pnpm's content-addressable store does not reconstitute, so
its install script dies with node-pre-gyp: command not found. That failure masked
unrelated build errors across the whole workspace.

Candidates evaluated

lib	Node 26	Bun 1.4	notes
wrtc	✗	✗	unmaintained since 2023, no native binding
@roamhq/wrtc	✓	panic	unsupported uv function: uv_async_init (oven-sh/bun#18546)
node-datachannel	✓	✓	chosen
werift	✓	✓	pure TS, kept as fallback only

@roamhq/wrtc passes on Node and hard-crashes on Bun — a Node-only CI would have
greenlit it.

Decision: node-datachannel

• libdatachannel bindings; N-API 8, so ABI-stable across Node majors
• platform optionalDependencies, no install script — removes the failure class entirely
• 8.3 MB native binary vs 24 MB for the libwebrtc-based forks
• node-datachannel/polyfill implements the W3C API, so the swap is one import line

Scope

• Remove wrtc, @mapbox/node-pre-gyp, and the allowBuilds: wrtc entry
• Delete src/server/wrtc.d.ts (declare module "wrtc" typed the whole module any);
  udp.server.network.ts gets type-checked for the first time
• Add NAT support, since the lib swap is the moment to do it:
  • ICE_SERVERS — JSON array of STUN/TURN servers
  • ICE_PORT — fixed port + ICE UDP mux, so every peer shares one port instead of a
    random one each (this is what broke the multi-pod Kubernetes setup)
  • ADVERTISE_IP — public address written into typ host candidates; srflx
    candidates are left alone since STUN already resolved them

All three default to off; current behaviour is unchanged without them.