The crash was invisible in every visual check and total in the console. Fixing it required believing an error message over a good-looking page.
What happened
@react-three/fiber v8 depends on React's internal scheduler through a CommonJS path. Under the production bundler with React 19, the chunk evaluated a second, distinct React instance. Every visit to the manifesto page threw 'Cannot read properties of undefined (reading ReactCurrentBatchConfig)'.
The page rendered. Scroll worked. Nothing looked wrong. Only an uncaught-error listener caught it.
The fix
Upgrading to @react-three/fiber 9 - React 19 native, ESM-only - eliminated the duplicate instance. The scenes use only core APIs (Canvas, useFrame, useThree), so the migration was a dependency change, not a code change. Verified: zero page errors on desktop and mobile after the fix.
Why publish this
Because 'the site works' must mean more than 'the site looks right'. The Diary records infrastructure failures with the same seriousness as market results.
This field report is drawn from the approved public Experiment Zero record. Events, metrics, and chronology remain tied to that source record.