Skip VTR map updates during joinpoint reload

[MhaWay]

Fixes #862

Summary

• suppress viewed map updates while SaveAndReload is in progress
• reuse the existing Multiplayer.reloading flag instead of adding new state
• clear VTR resend bookkeeping on reset so the reloaded client starts from a clean sync state

Validation

• dotnet build Source/Client/Multiplayer.csproj -c Release

[Copilot]

Pull request overview

This PR addresses join point save/reload inconsistencies by preventing viewed-map tracking updates from being sent while SaveAndReload() is in progress, and by ensuring VTR map resend bookkeeping is fully reset when multiplayer stops.

Changes:

• Skip sending viewed-map (PlayerCount) updates while Multiplayer.reloading is true.
• Reset VTRSync.lastSentAtTick alongside lastMovedToMapId so a fresh session doesn’t inherit resend suppression state.

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