feat(session): add RequestMux for routing inbound requests by type

[floatdrop]

What

RequestMux is the request-stream counterpart of Demux. Today a server (the relay, or any session.Server-based app) hand-rolls an AcceptRequest loop + a big type switch + dispatch. RequestMux lets it register a handler per message.Type instead:

mux := session.NewRequestMux()
mux.Handle(message.TypeSubscribe, func(r *session.Request) {
	go func() {                       // long-lived stream → spawn (Run is synchronous)
		pub, err := r.AcceptSubscribe(nil)
		...
	}()
})
mux.Handle(message.TypePublishNamespace, func(r *session.Request) {
	_ = r.Reply(&message.RequestOK{}); _ = r.Stream.Close()
})
mux.OnUnknown(func(r *session.Request) { _ = r.RejectError(moqt.RequestNotSupported, "…") })
_ = mux.Run(ctx, server)

The API and concurrency contract deliberately mirror Demux:

• Handle(t message.Type, h) / OnUnknown(f) / Run(ctx, *Session) error.
• Registration is concurrency-safe and may happen while Run is executing (a server learns some types to serve only after startup).
• Dispatch is synchronous — a handler runs to completion before the next AcceptRequest, exactly like Demux.Run. A handler that keeps a request stream open for a subscription's lifetime spawns a goroutine.
• Unmatched type with no OnUnknown → REQUEST_ERROR NOT_SUPPORTED + FIN, so the stream can't leak.

Run surfaces AcceptRequest errors unchanged, with a doc note that the session-fatal ones (§10.1 Request-ID parity/monotonicity, token-cache faults) must be escalated by closing the session — this is the natural seam for the later "self-close on protocol violation" cleanup.

Why

Removes the boilerplate accept-loop/type-switch from every server built on the library, matching the ergonomics Demux already gives the subscriber/data side.

Relay adoption is deferred on purpose: the relay's dispatch also runs a per-type limiter and token verification before handing off, which needs a pre-dispatch hook rather than a like-for-like swap. Keeping that out of this PR keeps a hot path untouched and the change reviewable.

Testing

• TestRequestMuxRoutesByType — routing by type, a handler registered after Run starts, and OnUnknown for an unregistered type.
• TestRequestMuxDefaultRejectsUnhandled — no OnUnknown → requester receives REQUEST_ERROR NOT_SUPPORTED.
• ExampleRequestMux (compile-checked) + README row.
• go test -race ./pkg/moqt/session/..., full go test ./..., golangci-lint run, and the modernize standalone check — all clean.

🤖 Generated with Claude Code