Isolate RNG in the gravship launch ritual's reachability check

[cmlee119]

Fixes #967.

Problem

Building the gravship launch confirmation dialog decides which pawns can board, in Dialog_BeginRitual's constructor:

Dialog_BeginRitual..ctor -> CreateRitualRoleAssignments
  -> RitualRoleAssignments.PawnNotAssignableReason
  -> RitualBehaviorWorker_GravshipLaunch.PawnCanFillRole
  -> CanReachGravship -> GravshipUtility.TryFindSpotOnGravship
  -> Region.RandomCell -> Rand.Range        // consumes RNG

Building the dialog is UI work opened only by the issuing peer (the currentExecutingCmdIssuedBySelf gate in CancelDialogBeginRitual), so this RNG runs on just one peer and advances the shared stream there only. Spamming the launch button repeats it and the divergence accumulates into a desync (Random state from commands doesn't match). Measured: with nothing ticking, the gravship map alone was off by +11,617 rand iterations, all on the peer that pressed launch.

Fix

Wrap RitualBehaviorWorker_GravshipLaunch.CanReachGravship in Rand.PushState / PopState, matching the existing SeedPreceptComp_UnwillingToDo_Chance isolation in the same file for RNG that "can be called in interface". CanReachGravship returns a bool that does not depend on the RNG (it only searches for any allowed cell), and the real boarding calls TryFindSpotOnGravship outside this check, so isolating here leaves both the check result and determinism intact.

Testing

• Reproduced on vanilla Multiplayer + Odyssey (async time + multifaction, no third-party mods): spamming the launch button desyncs a client with Random state from commands doesn't match.
• With this fix, ~100 rapid clicks produce no desync.
• Source/Client builds clean (Release).

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Investigated and written with the help of Claude (Anthropic).