WIP: perf(netwatch): enable Apple UDP batch I/O

[ifdario]

Description

Enable noq-udp's Apple fast path for netwatch sockets when both sendmsg_x and recvmsg_x are available at runtime. This lets the existing UDP transport batch sends and receives on Apple platforms. Systems without both symbols keep the current per-packet path.

The fast-apple-datapath dependency feature is enabled explicitly because UdpSocketState leaves this path disabled until the socket owner opts in.

API Changes

None.

Notes & open questions

The Apple entry points are private APIs. The implementation checks both symbols with dlsym before calling the unsafe opt-in, matching noq-udp's runtime symbol resolution and fallback behavior.

Change checklist

• Self-review.
• Documentation updates, where relevant.
• Tests if relevant.
• All API changes documented.
• This PR isn't slop, and is carefully crafted to have the intended effect.
• cargo make passes locally. (cargo fmt, cargo check -p netwatch --all-targets, cargo test -p netwatch, and Clippy passed; cargo-make is not installed.)

[flub]

what's the state of the apple fast data path? When first introduced in upstream quinn-udp it had to be disabled again. But I think there might have been more work to make it safer since then? I'm also mildly surprised it does not yet enabled by default in noq-udp if it is safely supported, what does quinn-udp do there?

[ifdario]

We should actually wait for quinn-rs/quinn#2748 I didn't realized about the partial sends

[flub]

So, we're closing this for now? if we'll have to still explicitly enable it maybe turn it into an issue?

[ifdario]

Sure, we can also make an issue for later on