moq-native: advertise QUIC preferred_address in the server config

[kixelated]

Summary

• Adds preferred_v4: Option<SocketAddrV4> and preferred_v6: Option<SocketAddrV6> to moq_native::ServerConfig, exposed via CLI (--server-preferred-v{4,6}), env (MOQ_SERVER_PREFERRED_V{4,6}), and TOML ([server] preferred_v{4,6} = "...").
• Plumbs both into quinn::ServerConfig::preferred_address_v{4,6} inside QuinnServer::new. Quinn handles CID allocation, reset-token generation, and transport-parameter packing.
• Adds a cli_does_not_clobber_toml_preferred_addresses regression test in rs/moq-relay/src/config.rs to guard the TOML-to-CLI merge behavior documented in CLAUDE.md.

Why

The QUIC preferred_address transport parameter (RFC 9000 §9.6) lets a server advertise an alternate address that the client migrates to shortly after the handshake. This unlocks a clean BGP anycast deployment shape:

• Every instance announces a shared anycast /24 (handshake target).
• Each instance advertises its own per-host unicast IP as preferred_address.
• New clients route via anycast to the nearest POP; steady-state connections pin to the unicast IP and survive BGP reconvergence.
• An overloaded host can withdraw the anycast route from BGP without dropping its existing connections, because they are no longer using that address.

Supporting clients today: Chrome M131+ (on by default since November 2024, ~99% migration success per Google's measurements) and any native Quinn client (moq-cli, moq-ffi-based iOS/Android/Swift/Kotlin). Firefox via neqo has the implementation but I could not confirm the glue layer wires it through; Safari has no public confirmation.

Out of scope

• Documentation for the deployment pattern (Vultr / OCI BYOIP / BIRD config) lives in a follow-up PR.
• noq and quiche backends do not expose this API. If either field is set, those backends will silently ignore it. Same shape as the existing tls.root (mTLS) flag, which is also Quinn-only.
• Client-side happy-eyeballs and Windows server dual-stack are separate concerns.

Test plan

• just check passes (clippy, tests, docs, shear, sort, remark).
• New regression test cli_does_not_clobber_toml_preferred_addresses passes.
• Manual smoke (optional, reviewer may skip): run moq-relay --server-bind '[::]:4443' --server-preferred-v4 127.0.0.2:4444 against a loopback alias, connect with moq-cli, capture with tshark -d udp.port==4443,quic -V and confirm the handshake transport parameters contain the preferred address and the client sends a PATH_CHALLENGE to it.

(Written by Claude)

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 92a97c39-52d8-47ca-8ae5-c05abc00943c

📥 Commits

Reviewing files that changed from the base of the PR and between 1430897 and ef5568d.

📒 Files selected for processing (3)

• rs/moq-native/src/quinn.rs
• rs/moq-native/src/server.rs
• rs/moq-relay/src/config.rs

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Walkthrough

This PR adds support for configuring QUIC's preferred_address transport parameter (RFC 9000 §9.6). Two new optional fields—preferred_v4 and preferred_v6—are added to ServerConfig, accepting IPv4/IPv6 socket addresses via CLI flags and environment variables. The Quinn backend conditionally sets these addresses in the QUIC transport parameters during server initialization. A regression test ensures that TOML-configured addresses are not overwritten by the configuration merge process when environment variables are absent.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main change: adding support for advertising QUIC preferred_address in the server configuration.
Description check	✅ Passed	The description is comprehensive and directly related to the changeset, explaining the motivation, implementation details, scope, and test plan.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch claude/awesome-diffie-642dd2

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