A friend on Chrome hit "C.subtrees is not iterable" — Chrome was serving
the previous skill-tree JSON shape ({nodes, edges} at top level) out of
the disk cache while running the new bundle that expects {subtrees}.
Hard reload fixed his session but other users could hit the same.
Three layers of defense:
1. backend/data.controller.ts: Cache-Control switches from
'public, max-age=3600' to 'no-cache, must-revalidate' so the browser
always revalidates JSON files against the server.
2. frontend/data.ts: every data fetch gets ?v=2 + cache:'no-cache' to
bust any existing browser/CDN entry.
3. frontend/data.ts loadSkillTree(): if the response still has the legacy
shape (no subtrees, but nodes array), wrap it in a single anonymous
subtree so the UI doesn't crash on iteration.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>