Support QRESYNC - Quick Mailbox Resynchronization (RFC 7162)

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Summary

Implement RFC 7162 QRESYNC (Quick Mailbox Resynchronization), allowing clients to efficiently resynchronize mailbox state after reconnection without fetching all messages.

Motivation

QRESYNC is the highest-impact extension for mobile clients. iOS Mail uses QRESYNC to avoid re-downloading the full mailbox state on every connection, dramatically reducing bandwidth usage and sync time on cellular connections.

Scope

• Advertise QRESYNC in CAPABILITY response (requires ENABLE activation)
• Support SELECT/EXAMINE with QRESYNC parameters: (UIDVALIDITY MODSEQ [known-uids [seq-match-data]])
• Return * VANISHED (EARLIER) <uid-set> for expunged messages since last sync
• Replace individual * n EXPUNGE with * VANISHED <uid-set> for QRESYNC-enabled sessions
• Track and persist per-mailbox HIGHESTMODSEQ (builds on CONDSTORE Support CONDSTORE - RFC4551 #419)
• Support UID FETCH with VANISHED modifier

Prerequisites

• ENABLE (#TBD) — QRESYNC requires ENABLE QRESYNC activation
• CONDSTORE (Support CONDSTORE - RFC4551 #419) — QRESYNC depends on MODSEQ tracking

References

• RFC 7162
• Depends on: CONDSTORE (Support CONDSTORE - RFC4551 #419), ENABLE