[BUG] `readonly` array modifier lost during type transformation

[kakasoo]

Bug Report

Description

In DeepDateToString, DeepStrictMerge, DeepMerge, and DeepStrictUnbrand, when a readonly array is provided as input, the readonly modifier is lost in the output type. Since type safety is the core focus of this library, loss of modifier information is a bug.

Type Issue Example

When a readonly array is provided as input, the output is converted to a mutable Array.

Type Input

// === DeepDateToString ===
type Input1 = readonly { date: Date }[];
type Result1 = DeepDateToString<Input1>;
// Expected: readonly { date: string }[]
// Actual:   { date: string }[]  (readonly lost)

// === DeepStrictMerge ===
type Target = readonly { a: number }[];
type Source = readonly { b: string }[];
type Result2 = DeepStrictMerge<Target, Source>;
// Expected: readonly { a: number; b: string }[]
// Actual:   { a: number; b: string }[]  (readonly lost)

// === DeepStrictUnbrand ===
type Input3 = readonly { a: number & { __brand: 'ID' } }[];
type Result3 = DeepStrictUnbrand<Input3>;
// Expected: readonly { a: number }[]
// Actual:   { a: number }[]  (readonly lost)

Affected Files

File	Line	Issue
src/types/DeepDateToString.ts	29-30	Only matches Array<infer I>, no readonly array branch
src/types/DeepStrictMerge.ts	13-15	Only matches Array<infer TE>, no readonly array branch
src/types/DeepMerge.ts	66-73	Same pattern
src/types/DeepStrictUnbrand.ts	56-68	Only matches Array<infer I>, no readonly array branch

Fix

Add a readonly (infer I)[] branch after each Array<infer I> branch to handle readonly arrays separately. This pattern is already used in DeepStrictObjectKeys:

// DeepStrictObjectKeys.ts:109 (existing reference pattern)
: DeepStrictUnbrand<Target> extends readonly (infer Element)[]

Test Requirements

All changes must include the following tests:

1. Backward Compatibility

   • All existing mutable array tests must pass (npm run build:test && npm run test)
   • Add tests to verify that existing type behavior remains unchanged

2. Fix Verification

   • For each type, verify readonly is preserved when inputting a readonly array using the Equal<Question, Answer> pattern

3. Complex Type Stability

   • readonly { nested: { deep: Date } }[] (readonly + nesting + Date)
   • readonly [{ a: 1 }, { b: 2 }] (readonly tuple)
   • readonly { a: number & MinLength<1> }[] (readonly + branded)
   • readonly (readonly { a: 1 }[])[] (2D readonly array)
   • readonly { items: readonly { date: Date }[] }[] (nested readonly arrays)

How to verify:

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