Fix Windows path resolution for normalization_path

[AnnihilatorChess]

Summary

The normalization path refactor introduced in #69 breaks on Windows due to a platform-specific path separator issue.

.lstrip("./") on this line only strips Unix-style separators (. and /). On Windows, os.path.join produces backslash-separated paths, so .removeprefix(trunk_path) leaves a leading \ (e.g. \stats.yaml), which .lstrip("./") does not strip. When this is passed to os.path.join, Windows interprets the leading \ as a drive-root path, resolving to C:\stats.yaml instead of the correct dataset-relative path.

Fix

Adding \ to the lstrip characters: .lstrip("./\\") — this has no effect on macOS/Linux since backslashes don't appear in paths there.

Steps to reproduce

1. On Windows, configure well_base_path as a relative path (e.g. ../../datasets/)
2. Run training with use_normalization=True
3. normalization_path incorrectly resolves to C:\stats.yaml