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>

scripts/steps_to_parquet.py

@@ -9,11 +9,14 @@ Usage:
 
 import argparse
 from pathlib import Path
+from typing import Literal
 
 import awkward as ak
 import polars as pl
 import uproot
 
+ParquetCompression = Literal["lz4", "uncompressed", "snappy", "gzip", "brotli", "zstd"]
+
 
 def _batch_to_polars(batch: ak.Array) -> pl.DataFrame:
     """Convert one awkward-array batch to a Polars DataFrame.
@@ -37,7 +40,7 @@ def convert_steps_to_parquet(
     output_path: str | Path | None = None,
     batch_size: str = "100 MB",
     tree_name: str = "Steps",
-    compression: str = "snappy",
+    compression: ParquetCompression = "snappy",
 ) -> Path:
     """Read *tree_name* from *root_path* and write it to a Parquet file.
 
@@ -108,7 +111,7 @@ def main() -> None:
         output_path=args.output,
         batch_size=args.batch_size,
         tree_name=args.tree,
-        compression=None if args.compression == "none" else args.compression,
+        compression="uncompressed" if args.compression == "none" else args.compression,
     )