Add mutual invisibility for vanished staff

[Zaldaryon]

Fixes #213.

What

/vanish hideothers on|off|toggle, a per-viewer preference. When on, a staff member stops seeing other vanished staff, without touching what anyone else sees. Also /vanish status, prints your own vanish state and this preference, changes nothing.

/vanish                          unchanged, toggle own vanish
/vanish status                   new
/vanish hideothers on|off|toggle new

New config default, Commands.VanishHideOtherVanishedDefault (bool, default false), the starting value for a staff member who never touched the preference. Default keeps today's behavior, vanish holders still see each other unless someone opts in.

Where it hooks and why

ShouldHideEntityFromClient(entity, client) already takes the viewer, not just the subject. The whole feature is one added branch there: non-staff still get an unconditional true, staff now return their own HidesOtherVanished(viewer.PlayerUID) instead of the old unconditional false. Nothing else calls this predicate directly, so no call site needed to change.

The tracking sweep in PhysicsManager.cs.patch won't despawn a hidden entity on its own at the range this issue is actually about. It has a hysteresis window, an already-tracked entity stays tracked as long as it's inside 1.21x the tracking radius, which is exactly two staff standing next to each other. So the toggle needs its own packet push or it silently does nothing at close range while appearing to work at long range. RefreshVanishedVisibilityForViewer is that push, the inverse of the existing HideVanishedPlayerFromOthers/RevealPlayerToOthers: those loop every client for one subject, this loops every vanished subject for one client.

HandleNear had its own inline copy of the old visibility rule (IsVanished(...) || PlayerHasAccess(...)), so it would have kept listing a staff member the viewer just chose to stop seeing. Updated it to match.

Testing

No test project in this repo, same live-boot convention as always.

• dotnet build, zero warnings on the touched files.
• Fresh boot reaches RunGame, no fatal errors.
• Live test, one connected account (a second wasn't available this round): /vanish, /vanish status, /vanish hideothers on, status again to confirm the flip, /near with nothing to hide (no crash), /vanish hideothers off, then bare /vanish to confirm it still toggles vanish and not the preference. stratum-audit.log shows vanish hideothers=on actor=Zaldaryon, confirming the command path end to end.

What that doesn't cover: the actual mutual-invisibility effect needs two vanished staff who can see each other's screens, and RefreshVanishedVisibilityForViewer's despawn push is exactly the part most likely to be wrong in a way single-account testing can't catch (see the hysteresis note above, wrong and it's an invisible bug at close range and a working one at long range). Worth a two-account pass before merge if that's easy on your end, happy to do it myself if a second account gets easier to arrange here.

@trevorftp @tehtelev