Issue: Implement GwStems manager for stems handling

Issue: Implement GwStems manager for stems handling: a task in LegoFlow-SWE (Harbor dataset). Currently, library functions rely on global variables for managing file/line “stems” during FST loading. To decouple stems from the FST loader and allow reuse, we need a new public type, GwStems , that…

The task

Currently, library functions rely on global variables for managing file/line “stems” during FST loading. To decouple stems from the FST loader and allow reuse, we need a new public type, `GwStems`, that encapsulates the addition of source file paths, stem entries, and “istem” entries, along with querying.

Part of Lego-X/LegoFlow-SWE.