## Depth Pooling Fixes
- Fixed `np.errstate` usage: `all_nan` is not a valid parameter for `errstate`. Changed to `invalid="ignore"`.
- Fixed `conf_stack` possibly unbound error by initializing it to `None` and checking it before use.
- Removed duplicated unreachable code block after the first `return`.
- Fixed implicit string concatenation warning in `ValueError` message.
- Updated type hints to modern Python style (`list[]`, `|`) and removed unused `typing` imports.
- Verified with `basedpyright` (0 errors).