When a module script (e.g. canvas-*.js) fails to load (502 during
deploy, network error), the pane stayed in cache with a blank canvas.
Subsequent tab switches showed the broken cached pane instead of
re-fetching. Now script onerror removes the failed tag and evicts
the pane, so the next switchTo does a fresh fetch.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
