Feature: Core Constraint Helpers

[Goldziher]

Focused Scope

Add small sync constraint helpers for non-Zod factories if there is still demand after Zod and JSON Schema coverage.

Proposed v1

• unique
• min
• max
• between
• pattern
• custom
• Factory-local uniqueness only.
• A small ConstraintError with field, constraint, and attempts where applicable.

Out of Scope

• Joi/Yup integrations.
• Redis/database-backed uniqueness.
• Async validators.
• Retry strategy DSLs.
• Conditional validation frameworks.

This should not duplicate ZodFactory or JsonSchemaFactory; it should only cover lightweight core factory constraints.