Adopt sliding sync for large accounts

[Deniel9204]

The client uses full /sync; adopt sliding / simplified sliding sync (MSC4186/3575) for fast startup and lower memory on large accounts.

Type: feature · Priority: P2 · Area: performance

Related upstream issues: none (parity/architecture gap)

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[Deniel9204]

Closing as not feasible in this SDK family. Upstream famedly/matrix-dart-sdk has no sliding-sync implementation (0 code matches, including latest v7.4.0 — sliding sync lives in the Rust SDK, not the Dart one), and the commetchat fork we depend on is 18 commits ahead of famedly v6.1.1 with load-bearing patches (background-service support + VoIP fixes). Switching to upstream famedly would break background service + calling and still not provide sliding sync. See docs/session-summary-2026-06.md.

[Deniel9204]

Path forward (researched): no production Dart/Flutter Matrix SDK implements sliding sync today. famedly matrix is /sync v2 only through v7.4.0; native MSC4186 exists only as WIP draft famedly PR #2198. matrix-rust-sdk has MSC4186 (Element X) but only Swift/Kotlin bindings — a Flutter integration would be a DIY FFI bridge (multi-month, ~rewrite). Recommended: track/sponsor PR #2198 rather than an FFI bridge. Documented in docs/session-summary-2026-06.md.