Feature: Core Constraint Helpers

## 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.