fix(EntityEnhancements): guard eye detours against re-entrancy (StackOverflow crash)

[Prefix]

The four PointViewControl eye detours resolve managed wrappers while running inside the hooked native function:

• entry: _entityManager.MakeEntityFromPointer<IPlayerPawn>(pEntity)
• ShouldOverrideEye() → pawn.GetCameraService()?.ViewEntity → BaseEntity.Create()

If any of that work reaches the hooked function again, the detour re-enters itself and recurses until the stack is exhausted. The process then dies with System.StackOverflowException, which .NET does not permit catching — the server goes down with no recoverable error and no log line.

Evidence

Two managed crash reports from a production server, six days apart, identical signature:

System.StackOverflowException
  Sharp.Modules.EntityEnhancements.Modules.PointViewControl.LinuxGetEyePosition(IntPtr)
    Sharp.Core.Managers.EntityManager.MakeEntityFromPointer<__Canon>(IntPtr)
      Sharp.Core.GameEntities.BaseEntity.Create(IntPtr)
        "repeated": "0x4340"        <- 17,216 frames

The second report shows 17,208 frames of the same function. Both are on a build that already contains the vtable-resolved hook from #109, so that change does not address this.

Note ShouldOverrideEye's second operand is evaluated on every call whenever _viewControlEntities is empty — i.e. on every server that has no point_viewcontrol entity but does call GetEyePosition.

Why it is rarely seen

The hook installs unconditionally at Init(), but the detour body only runs when something actually calls GetEyePosition/GetEyeAngles. Servers whose plugins never call them never execute it. It reproduces reliably on a server whose plugins call IBasePlayerPawn.GetEyePosition() frequently.

Fix

A [ThreadStatic] re-entrancy flag — if the detour is already active on this thread, go straight to the trampoline.

• thread-static because the hooked function is not main-thread-only
• try/finally so the flag is cleared even if wrapper resolution throws; MakeEntityFromPointer throws InvalidOperationException("nullptr") when the pointer does not resolve, which would otherwise latch the flag permanently
• applied to all four detours (Linux + Windows, position + angles) since they share the same shape

Behaviour is unchanged for every non-re-entrant call. Builds clean against master (0 Warning(s), 0 Error(s)).

Possible follow-up (not in this PR)

The hook is installed even when the map contains no point_viewcontrol entity, so on most servers it is pure overhead on a hot path. Deferring installation until the first such entity spawns would avoid the cost entirely — happy to do that separately if you would like it.