fix(form-core): support value types with a custom equals() method in deep equality

[FrancoKaddour]

Closes #2195

Problem

evaluate (the deep-equality helper used for dirty-checking) reports two equal value-type instances as unequal. Value types like Temporal.PlainDate, Luxon DateTime, etc. expose their state through private fields/getters, so they have no own enumerable keys and a non-plain prototype — which hits the no-enumerable-keys guard added in #2140 and returns false. A field backed by such a value is then considered dirty even after it's reset to its original value.

Fix

Before that guard, delegate to a custom equals() method when both operands share a constructor that exposes one. This covers Temporal types, Luxon DateTime, Immutable collections, and any value type following the equals() convention, while leaving plain objects/arrays and the existing Date/Map/Set cases untouched.

Tests

Added a form-core unit test using a value type with a private field and an equals() method (mirroring Temporal's shape — no own enumerable keys). Reverting the fix fails it; the full form-core suite passes, plus typecheck and lint.

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