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_flow.py

@@ -1,8 +1,7 @@
 import torch
-import pytest
 from giant.model.network import DenoisingMLP
 from giant.model.schedule import CosineSchedule, flow_matching_loss
-from giant.sample import sample_flow, sample_ddpm, sample_ddim
+from giant.sample import sample_flow, sample_ddim
 
 
 def _small_model():