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Fixes #17002

Problem

Array.isArray() narrows to arg is any[], which silently drops the readonly modifier when the input type already carries it:

function f(x: readonly string[] | string) {
    if (Array.isArray(x)) {
        x; // narrowed to string[] — readonly is silently dropped
        x.push("oops"); // no error, but the value was declared readonly
    }
}

This has been open for 9 years with significant community interest (64 comments).

Fix

One additive overload in src/lib/es5.d.ts:

isArray(arg: readonly any[] | any): arg is readonly any[];  // NEW
isArray(arg: any): arg is any[];                            // existing, unchanged

Overload resolution picks the first matching signature, so:

Backward compatibility

A concern raised in #17002 (by @sisp) is that changing isArray to return readonly unknown[] for unknown inputs would be a breaking change. This PR does not have that problem — the new overload only matches when the input type is already readonly any[] | T. For unknown, any, or any mutable array type, overload resolution falls through to the existing isArray(arg: any): arg is any[] signature, so behavior is identical to today.

Scope

• 1 line added to src/lib/es5.d.ts (additive overload)
• 1 test file added: tests/cases/compiler/arrayIsArrayReadonlyNarrowing.ts
• No changes to checker logic, no changes to existing overload behavior
• Zero risk to existing code — the only new behavior is preserving readonly when it was already there

Submitted as a draft to get early feedback on whether this overload shape is the right approach, or whether the team prefers handling this differently (e.g. conditional type, checker change).
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