[iOS] Tune background-sync timeout against iOS 26 background-grant tightening

[pvg13]

Context

The APNs-wake background sync uses a 25s timeout (WaveSyncPushHandler → wavesync_background_sync_targeted(..., 25, ...)), chosen to fit inside the historical ~30s background grant. iOS 26 tightened background execution, so the grant may occasionally be shorter.

Task

• Confirm on-device how much background time the silent-push wake actually gets on iOS 26.
• Keep the timeout conservative; ensure a truncated / partial sync returns gracefully (UIBackgroundFetchResult) and relies on the next foreground resume (already wired via the lifecycle observer).
• Adjust the 25s value if measurements warrant.

References

• Gap P2-8 / Open Question Shadow-table errors don't halt sync dispatch #7 in docs/research/ios-sync-state-of-the-art.md