fix(tutorial): /tutorial/update preserves max TutorialState

The endpoint used to write the client-supplied step verbatim, so a stale or
replayed request with tutorial_step=0 against any later-stage viewer would
regress the persisted state to 0. NextSceneSwitcher routes step==0 to
AreaSelect section 0, which has no chapter data — the client LINQ-Single()
crashes on next /load/index, bricking the viewer. Math.Max-preserve matches
the 31→41 pattern in GiftController.TutorialGiftReceive.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/TutorialController.cs

@@ -36,7 +36,14 @@ public class TutorialController : SVSimController
             .Include(v => v.MissionData)
             .FirstAsync(v => v.Id == viewerId);
 
-        viewer.MissionData.TutorialState = request.TutorialStep;
+        // Preserve max — never regress. Mirrors GiftController.TutorialGiftReceive's 31→41 guard.
+        // Without this, a stale or replayed request with tutorial_step=0 (or any value below the
+        // viewer's current state) crashes the client on next /load/index: NextSceneSwitcher routes
+        // step==0 to AreaSelect section 0, which has no chapter data → LINQ Single() failure.
+        // Response keeps echoing request.TutorialStep so the client's own transition confirmation
+        // still works; the client owns the step-it-thinks-it's-moving-to concept and we don't
+        // want to surface a divergent value mid-flow.
+        viewer.MissionData.TutorialState = Math.Max(viewer.MissionData.TutorialState, request.TutorialStep);
         await _db.SaveChangesAsync();
 
         return new TutorialUpdateResponse { TutorialStep = request.TutorialStep };