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>
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