feat: Comparer interface for custom Equal assertions

[sonnemusk]

Summary

Adds assert.Comparer so domain types can define equality for Equal / NotEqual (and anything else using ObjectsAreEqual) without reflect.DeepEqual field-by-field noise (issue #1204).

type Money struct {
    Cents int
    Label string // display only
}

func (m Money) Equal(other interface{}) bool {
    o, ok := other.(Money)
    return ok && m.Cents == o.Cents
}

assert.Equal(t, Money{100, "$1.00"}, Money{100, "1 dollar"}) // true

Behavior

• If expected implements Comparer, use expected.Equal(actual)
• Else if actual implements Comparer, use actual.Equal(expected)
• Else existing []byte / reflect.DeepEqual path

No codegen changes; only ObjectsAreEqual grows a short interface check.

Fixes #1204