Add tooling for a versioned geant_steps dataset layout

Introduces raw/<kind>/<gen>/<detector>/shard-NNN.root and
processed/<kind>/<gen>/<schema>/<detector>/shard-NNN.parquet as the dataset
convention, plus scripts to operate on it: migrate_geant_steps.py for the
one-time move into this layout, bump_dataset_version.py to cut new
gen/schema versions with a logged reason, steps_to_parquet_parallel.py to
convert ROOT shards to parquet in parallel and place them correctly, and
create_root_files.py to generate new ROOT shards via a minicalosim
executable. The loader gains .manifest file support so pools/ (train/dev/
holdout shard lists) can be passed straight to `giant train`.

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

tests/test_loader.py

@@ -0,0 +1,60 @@
+import pytest
+
+from giant.data.loader import find_parquet_files
+
+
+def _touch(path):
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.touch()
+    return path
+
+
+def test_find_parquet_files_single_file(tmp_path):
+    f = _touch(tmp_path / "shard-000.parquet")
+    assert find_parquet_files(f) == [f]
+
+
+def test_find_parquet_files_directory_glob(tmp_path):
+    a = _touch(tmp_path / "shard-000.parquet")
+    b = _touch(tmp_path / "shard-001.parquet")
+    _touch(tmp_path / "not_a_parquet.root")
+    assert find_parquet_files(tmp_path) == sorted([a, b])
+
+
+def test_find_parquet_files_empty_directory_raises(tmp_path):
+    with pytest.raises(FileNotFoundError):
+        find_parquet_files(tmp_path)
+
+
+def test_manifest_resolves_relative_to_its_own_directory(tmp_path):
+    target = _touch(tmp_path / "processed" / "pbwo4" / "shard-000.parquet")
+    manifest_dir = tmp_path / "pools" / "pbwo4"
+    manifest_dir.mkdir(parents=True)
+    manifest = manifest_dir / "full.manifest"
+    manifest.write_text("../../processed/pbwo4/shard-000.parquet\n")
+
+    assert find_parquet_files(manifest) == [target.resolve()]
+
+
+def test_manifest_skips_blank_lines_and_comments(tmp_path):
+    target = _touch(tmp_path / "shard-000.parquet")
+    manifest = tmp_path / "full.manifest"
+    manifest.write_text("\n# a comment\nshard-000.parquet\n\n")
+
+    assert find_parquet_files(manifest) == [target.resolve()]
+
+
+def test_manifest_missing_file_raises(tmp_path):
+    manifest = tmp_path / "full.manifest"
+    manifest.write_text("does_not_exist.parquet\n")
+
+    with pytest.raises(FileNotFoundError):
+        find_parquet_files(manifest)
+
+
+def test_manifest_with_no_entries_raises(tmp_path):
+    manifest = tmp_path / "full.manifest"
+    manifest.write_text("# only comments\n")
+
+    with pytest.raises(FileNotFoundError):
+        find_parquet_files(manifest)