fix: copy all Differ options in structValues to preserve TagName

[Yanhu007]

Fixes #117

Problem

structValues creates a new Differ for recursive diffing but only copies Filter and customValueDiffers. All other configuration fields — notably TagName, SliceOrdering, FlattenEmbeddedStructs, etc. — are lost.

This means when diffing nested or embedded structs via CREATE/DELETE paths, tag names are ignored and raw field names are used instead.

type A struct {
    Field1 struct {
        Field2 string `diff:"F1"`
    } `diff:"F2"`
}
// Expected path: ["F2", "F1"]
// Actual path:   ["F2", "Field2"]  (F1 tag lost in nested struct)

Fix

Copy all Differ configuration fields when creating the nested instance in structValues, using a struct literal instead of zero-value + selective assignment.

All existing tests pass.