fix(schema): make visible optional for validation

[EfeDurmaz16]

Summary

• mark renderer visible fields as explicitly optional in schema definitions
• update spec type inference so s.optional() fields are optional in inferred spec types
• add regression coverage for validating specs that omit visible

Fixes #287.

Verification

• pnpm exec vitest run packages/core/src/schema.test.ts
• pnpm type-check
• temporary local smoke with @json-render/core using zod@4.4.3: pnpm exec vitest run packages/core/src/schema.test.ts

[vercel]

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