Add ruff and ty as dev dependencies, fix lint/type findings

ruff removed unused imports across analysis.py and several test files.
ty caught a wrong dict[int, int] annotation on StreamingStepsDataset's
mat_map (materials are strings) and a real bug in steps_to_parquet.py
where --compression none passed None to polars' write_parquet, which
only accepts the literal "uncompressed". Also narrows a few
Optional-typed attributes (ddpm_schedule, Normalizer.mean/std) with
asserts and aligns __getitem__'s parameter name with torch's Dataset
base class.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_analysis.py

@@ -3,7 +3,6 @@ import matplotlib
 matplotlib.use("Agg")  # no display needed for plot smoke tests
 
 import numpy as np
-import pandas as pd
 import pyarrow as pa
 import pyarrow.parquet as pq
 import pytest