fix(v4): clone Map and Set in shallowClone to prevent shared state

[artur-seppa]

Problem

When using .default() with mutable values like Map or Set, every call to
.parse(undefined) returns the same reference. Mutations on one parse result
leak into subsequent parses:

const schema = z.map(z.string(), z.number()).default(new Map());

const result1 = schema.parse(undefined);
result1.set("key", 42);

const result2 = schema.parse(undefined);
console.log(result2.size); // 1 — should be 0

This already worked correctly for plain objects and arrays via shallowClone,
but Map and Set fell through to return o (same reference).

Fix

Extended shallowClone in util.ts to cover Map and Set:

if (o instanceof Map) return new Map(o);
if (o instanceof Set) return new Set(o);

Tests

Added four tests to default.test.ts:

shallow clone returns distinct instances for Map and Set
mutations on one parse result do not affect another (both directions)
Fixes #5826

[colinhacks]

@pullfrog review

[pullfrog]

Reviewed — no issues found.

Task list (5/5 completed)

• Read the PR diff
• Review util.ts changes (shallowClone)
• Review test changes
• Check for edge cases and correctness
• Submit review

  ^{｜ View workflow run ｜ Using Claude Opus ｜ 𝕏}

[colinhacks]

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