WIP: perf(datagrams): expose batch send/recv APIs through iroh Connection

[ifdario]

Description

Expose the new noq batch datagram APIs through iroh's Connection wrapper so downstream crates (e.g. rayfish) can amortize the per-datagram connection-mutex cost, which is the dominant overhead at high pps for a mesh VPN forwarding bursts of TUN packets.

• Connection::send_datagrams / read_datagrams delegate to noq::Connection.
• Re-export ReadDatagrams and SendMany through endpoint::quic and the public endpoint surface (required by the crate's unreachable_pub lint).

Depends on the noq PR that adds Connection::send_datagrams / read_datagrams and DatagramState::recv_many / Datagrams::send_many. This branch should not merge until that noq release is published and the [patch.crates-io] override (see below) is removed.

Breaking Changes

None. All additions are additive methods and types. Existing send_datagram / read_datagram / send_datagram_wait are unchanged.

Notes & open questions

• This branch currently carries a [patch.crates-io] override in the workspace Cargo.toml pointing noq / noq-proto / noq-udp at a local fork (../noq) so iroh can see the new APIs during development. This MUST be removed before merge, which requires the noq batch-API PR to be merged and a new noq release published to crates.io first. Leaving the patch in would break CI for anyone without the local ../noq checkout.
• The noq dependency version in iroh/Cargo.toml is not yet bumped; a follow-up commit will bump noq (and noq-proto / noq-udp) to the released version that contains the batch APIs, and remove the patch, in the same change.
• No new tests in iroh itself. The underlying APIs are tested in noq; iroh's wrappers are thin passthroughs, so the existing iroh datagram tests continue to exercise the single-datagram path. Adding an iroh-level integration test for the batch passthrough is possible but deferred to avoid duplication.
• No benchmark delta. iroh's internals do not use the batch APIs yet, so iroh's own performance is unchanged. The win is realized by downstream callers that batch (e.g. rayfish's TUN read loop). Measuring that is left to the downstream caller.

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant. (deferred, see Notes)
• All breaking changes documented. (none)

[matheus23]

Needs paths to be updated

[ifdario]

I'll do it once we merge noq. Then we can upgrade to whatever version that one adds

[ifdario]

Related: n0-computer/noq#735

[flub]

Thanks for taking care of this side as well! But let's move this back into draft until n0-computer/noq#735 is merged. Please re-open once that's been merged.