[Due for payment 2026-06-26] [No QA] Add SEQUENTIAL_QUEUE.md

[mountiny]

Problem

The SequentialQueue subsystem — the offline-first WRITE queue that serialises every mutating API call — lacked a dedicated reference document. Engineers had no single observational guide covering the coordinator, persisted store, throttling, anti-flicker buffers, middleware boundary, and known sharp edges.

Solution

Added contributingGuides/SEQUENTIAL_QUEUE.md, mirroring the style of NETWORK_STATE_DETECTION.md. The doc explains how SequentialQueue works today (state machine, PersistedRequests in-memory-authoritative pattern, RequestThrottle backoff, QueuedOnyxUpdates, offline/pause/multi-tab behaviour, conflict resolution, and test coverage) and includes an Open Questions appendix for intent-ambiguous items needing maintainer confirmation.

PR

#93279

Issue Owner

Current Issue Owner: @adhorodyski

[melvin-bot]

Reviewing label has been removed, please complete the "BugZero Checklist".

[melvin-bot]

The solution for this issue has been 🚀 deployed to production 🚀 in version 9.4.15-3 and is now subject to a 7-day regression period 📆. Here is the list of pull requests that resolve this issue:

• [No QA] Add SEQUENTIAL_QUEUE.md #93279

If no regressions arise, payment will be issued on 2026-06-26. 🎊

For reference, here are some details about the assignees on this issue:

• @adhorodyski does not require payment (Contractor)

[melvin-bot]

@adhorodyski Uh oh! This issue is overdue by 2 days. Don't forget to update your issues!