moq-tokio does not compile with --no-default-features, and workspace feature unification hides it

[kixelated]

cargo check -p moq-tokio --no-default-features fails on dev. Found while reviewing #2977; the breakage predates #2921.

Errors

At ca2fd504f (before #2921 merged), 4 errors:

• 2x E0004: non-exhaustive patterns: type &RequestKind is non-empty (all variants cfg'd out by the missing backend features, so the remaining matches don't compile)
• 2x E0282: type annotations needed

#2921 added a 5th: E0433: cannot find util in crate from worker.rs, which calls crate::util::resolve while mod util is gated on any(noq, quinn, quiche). worker::Workers::bind now also references the equally-gated crate::server::DEFAULT_BIND (#2977), deepening the same dependency rather than introducing it.

Why CI never sees it

Nightly just rs features runs --no-default-features across the workspace, and cargo unifies features per crate: moq-relay (and everything else) depends on moq-tokio with a backend enabled, so moq-tokio never actually compiles with zero features in a workspace build. Only a -p moq-tokio --no-default-features invocation, or an external consumer with default-features = false and no backend feature, hits it.

Options

• Decide the backend-less configuration is unsupported and enforce it: compile_error! when no QUIC backend feature is enabled (a tcp/uds-only build would need its own thought), which turns 5 confusing errors into 1 honest one.
• Or make it compile: ungate util/DEFAULT_BIND (both are dependency-free), gate worker/steer on a backend, and fix the pre-existing RequestKind matches.

The first is less work and matches reality: worker, steer, and server all assume a backend exists.

(written by Fable 5)