docs: P5-42 done + final Tier C dispositions (P5-51/52/53)

- P5-42 → [~] IMPLEMENTED (pragmatic WebView2 keep-alive) + LOTUS_FEATURES entry.
- P5-51 → [DEFERRED] with a concrete future-work spec (single-session storage map:
  sessions.ts localStorage keys + initMatrix IndexedDB stores; the 6 things true
  per-context isolation needs; multi-account as the smaller intermediate step).
- P5-52 → [DROPPED] (matrix-js-sdk can't do true per-room sync filtering; only
  cosmetic client-side hiding).
- P5-53 → [DEFERRED] with the lighter automation-rules alternative recorded.

Every desktop P5 item is now dispositioned: implemented, won't-fix, or
deferred-with-spec/dropped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

LOTUS_FEATURES.md

@@ -1186,6 +1186,10 @@ Exposes call status + a mute control to the Windows volume-flyout / media overla
 
 Detects Windows connectivity changes (`INetworkListManager`) and nudges the Matrix client to reconnect (`retryImmediately`). `useTauriNetwork` ↔ `native/network.rs`.
 
+### Instant Background Sync (P5-42)
+
+Keeps the `/sync` loop + notifications running full-speed while the app is closed to the tray, by disabling Chromium background throttling via WebView2 `additional_browser_args` (`lib.rs`) — no separate background process. Windows/WebView2 only; doesn't block system sleep.
+
 ### Native Rich Notifications (P5-41 / P5-35)
 
 Windows toasts with **click-to-open-room** and **inline quick reply** (WinRT `ToastNotification`, in-process `Activated` event). Falls back to the standard toast otherwise. `useTauriToastActions` ↔ `native/toast.rs`; the desktop notification bridge routes room notifications to it.