refactor: merge Node into PatchBuilder for a fluent API

[brunoga]

This PR simplifies the manual patch construction API by merging the Node and PatchBuilder types into a single, fluent PatchBuilder[T].

Key changes:

• Navigation methods (Field, Index, etc.) now return a new *PatchBuilder[T] pointing to the sub-path.
• Operation methods (Set, Put, Add, Delete, etc.) now return the same *PatchBuilder[T] for chaining.
• Error handling is now stateful: errors are recorded in the builder and returned by Build().
• Removed the need for .Root() calls.

Example of new API:

b := deep.NewPatchBuilder[MyStruct]()
b.Field("Name").Set("old", "new")
b.Field("Tags").Add(0, "first")
patch, err := b.Build()