fix(build): pin zod to 4.3.6 to fix Nextra MDX prerender crash

zod 4.4.x tightened validation ('expected nonoptional, received undefined at
children') which crashes Nextra 4.6.1's Server Components prerender of MDX pages.
The known-good tree used zod 4.3.6; a from-scratch lockfile regen pulled 4.4.3.
Pin zod to 4.3.6 via overrides so any lockfile regen keeps the working version.
Revert the earlier next/nextra exact pins — next 16.2.10 was never the cause.

Author: boyander