feat(typedef): re-export Gen 4 layout engine types from lib.rs

Re-export ByteRange, OffsetMap, FieldPosition, LayoutBuilder, PackedLayout,
FieldValue, SequentialReader, and UnionDispatch at the crate root so consumers
can access the layout engine types without module-qualified paths. The
discriminator read functions (read_byte_discriminator, etc.) remain
module-qualified under alknet_typedef::tunion.

crates/alknet-typedef/src/lib.rs

@@ -36,4 +36,8 @@ pub use schema::{
     parse_align, parse_discriminator, parse_encoding, parse_endian, parse_max_length,
     normalize_refs, DiscriminatorKind, Endian, VariableEncoding,
 };
-pub use validation::build_validator;
+pub use validation::build_validator;
+pub use offset_map::{ByteRange, OffsetMap};
+pub use layout_builder::{FieldPosition, LayoutBuilder, PackedLayout};
+pub use sequential_reader::{FieldValue, SequentialReader};
+pub use tunion::UnionDispatch;