docs: three names in core.mdx don't exist in zod/v4/core

packages/docs/content/packages/core.mdx uses three names that don't exist in zod/v4/core (checked against 4.6.5):

• z.util.isValidJWT(...) → z.isValidJWT(...). isValidJWT is exported from the top level of zod/v4/core; util has no such member.
• z.min(5) → z.minLength(5). There is no min check factory.
• [$ZodCheckEmail, $ZodCheckMinLength] → [$ZodEmail, $ZodCheckMinLength]. No $ZodCheckEmail class exists; the email check is the $ZodEmail string-format schema.

z.string().check(z.email()).check(z.minLength(5))._zod.def.checks.map((c) => c.constructor.name)
// => ['ZodEmail', '$ZodCheckMinLength']

The three-line fix is on changbaebang/zod@c3ed77cded4f785d58282bd23121c735f2945192 (branch fix-core-mdx-api-names) if a PR is welcome — opening one is currently rejected for this account.