fix(auth): keep accepted grants on fixed expiry deadlines

[kixelated]

Problem

The auth client recomputes expiry from the wall clock each time its recheck loop runs, restarting the timer when the wall clock does not advance with Tokio. The relay also rejects an otherwise valid grant immediately when its expiry falls inside moq-auth's five-second skew allowance.

Approach

Share expiry conversion through Grant::deadline(), snapshot it once per accepted grant in the client and relay, and schedule client rechecks on Tokio's clock. Keep paused-clock HTTP fixtures on the same runtime and isolate their transport timeout from the lease clock.

Impact

• Add Grant::deadline() -> Optiontokio::time::Instant under the existing tokio feature; client and serve enable that feature.
• Past expiry within five seconds gets the remaining skew window on both sides; future expiry keeps its existing deadline.
• Lease holders continue to enforce fixed grants. No wire-format or grant JSON changes.

Alternatives

Moving clock ownership into lease::Producer would change fixed-lease behavior and require a broader API change. The approved additive helper keeps deadline ownership in the existing client and relay drivers.

Validation

• Before the fix, the relay skew regression failed and the paused client outage test missed expiry. With the fix, all five focused clock regressions pass.
• Core-only, tokio-only, and serve-only moq-auth library builds pass.
• nix develop --command just check: passed (527 default tests, 107 feature tests, docs, formatting, and packaging checks).

Follow-ups

None. Completes the auth-expiry-clock quest.

(Written by GPT-6)

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📥 Commits

Reviewing files that changed from the base of the PR and between b99cad9 and 15be2ce.

📒 Files selected for processing (8)

• doc/bin/relay/auth.md
• doc/lib/rs/moq-auth.md
• quest/m1/README.md
• quest/m1/auth-expiry-clock.md
• rs/moq-auth/Cargo.toml
• rs/moq-auth/src/client.rs
• rs/moq-auth/src/grant.rs
• rs/moq-relay/src/auth.rs

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[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-26T02:59:00.918948Z	15be2ce	Draft marked ready

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[kixelated]

Merge check: CI green, Codex approved with no findings, no conflicts with main. Docs in doc/lib/rs/moq-auth.md and doc/bin/relay/auth.md match the new Grant::deadline() behavior (additive, tokio feature). No overlap with #4244, which only uses future expiries. No changes needed; enabling auto-merge.

(Written by Claude Opus 5.5)