[FEAT] Add `DeepReadonly<T>` type

[kakasoo]

Feature Request

• Extensions of existing features
• Propose a type that didn't exist before

Type Expectation

A DeepReadonly<T> type is needed that recursively converts all properties of objects and arrays to readonly. Since this library provides "deep + strict" type manipulation, DeepReadonly is a natural extension.

Example Type

type Input = {
  a: number;
  b: {
    c: string;
    d: { e: boolean }[];
  };
};

type Result = DeepReadonly<Input>;
// Expected:
// {
//   readonly a: number;
//   readonly b: {
//     readonly c: string;
//     readonly d: readonly { readonly e: boolean }[];
//   };
// }

Proposed Solution

Create src/types/DeepReadonly.ts with the following rules:

1. Add readonly modifier to all properties
2. Convert arrays to readonly T[]
3. Do not recurse into Date types (existing library convention)
4. Preserve branded types (do not unbrand before processing)
5. Add re-export to src/types/index.ts

Use Case

• Enforce immutability on API response objects to prevent accidental mutation
• Guarantee readonly state in Redux/Zustand store types
• Provide type definitions for the deep version of Object.freeze

Test Requirements

All changes must include the following tests:

1. Backward Compatibility

   • All existing tests must pass; the new type must not affect existing types

2. Feature Verification

   • Simple object readonly conversion
   • Nested object readonly conversion
   • Array to readonly array
   • Objects inside arrays are also readonly
   • Date type preserved (not made readonly)
   • Already readonly types remain unchanged (idempotency)

3. Complex Type Stability

   • 3+ levels of nesting ({ a: { b: { c: { d: number } } } })
   • 2D arrays ({ matrix: number[][] })
   • Arrays inside objects inside arrays ({ items: { tags: string[] }[] })
   • Tuple types ([string, { a: number }])
   • Optional property preservation ({ a?: number } → { readonly a?: number })
   • Union types ({ a: string | null })
   • Branded types ({ id: number & { __brand: 'ID' } })
   • any, never, unknown properties

How to verify:

npm run build:test && npm run test
npm run prettier