PRD: MSC4186 v4 Sync - Critical Gap Implementation

PRD: MSC4186 v4 Sync - Critical Gap Implementation

Problem: Phase 11 functional testing revealed critical gaps in Dendrite's v4 sync implementation that block Element X client compatibility: required state filtering not implemented, account_data extension returns empty, is_dm filter/field not working, BumpStamp always 0, and E2EE fallback keys missing.

Solution: Implement P0-P3 gaps in priority order: (1) Required state filtering with wildcards, (2) account_data extension (global + per-room), (3) is_dm detection and filtering, (4) BumpStamp calculation, (5) E2EE fallback key types field.

Detailed PRD: prds/2-v4-sync-gap-implementation.md

Priority: P0 (Critical - Blocks Element X)