Add IEqualityComparer and Func predicate overloads to equality assertions

[Copilot]

All equality assertions (Equal, NotEqual, SequenceEqual, NotSequenceEqual) now support custom comparison via IEqualityComparer<T> or Func<T?, T?, bool> overloads.

Changes

• ObjectAssertions<T> — Equal and NotEqual overloads with comparer/predicate
• EnumerableAssertions<TEnumerable, T> — SequenceEqual and NotSequenceEqual overloads; extracted SequenceEqualCore to share logic across overloads
• ReadOnlySpanAssertions<T> — All four span and IEnumerable variants of SequenceEqual/NotSequenceEqual; extracted SequenceEqualEnumerableCore/NotSequenceEqualEnumerableCore

16 new public methods, 28 new tests (381 → 409).

Usage

// IEqualityComparer overload
"hello".Should().Equal("HELLO", StringComparer.OrdinalIgnoreCase);

// Func predicate overload
"hello".Should().Equal("HELLO", (a, b) => string.Equals(a, b, StringComparison.OrdinalIgnoreCase));

// SequenceEqual with comparer
new[] { "a", "b" }.Should().SequenceEqual(new[] { "A", "B" }, StringComparer.OrdinalIgnoreCase);

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