Hydration & Navigation Fixes for Deco TanStack Storefronts

Patterns and fixes for hydration mismatches, flash-of-white, CLS, scroll issues, and React warnings discovered in production Deco storefronts running TanStack Start + React 19 + Cloudflare Workers.

1. Flash-of-White / Blank Screen on F5

Symptom: Page loads, goes blank for a moment, then content reappears.

Root cause: React.lazy + <Suspense> for eager (above-the-fold) sections. Even with syncThenable optimization, the module may not be synchronously available during hydration, causing React to show the fallback and discard server HTML.

Fix: Synchronous Section Registry

Register critical above-the-fold sections with static imports so they never go through React.lazy: