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>

giant/data/loader.py

@@ -5,9 +5,33 @@ import numpy as np
 import pandas as pd
 import pyarrow.parquet as pq
 
+# A manifest is a plain text file listing one parquet path per line, used to
+# name a curated subset of files (e.g. a train/holdout pool) without copying
+# or symlinking the underlying parquet files. Lines are resolved relative to
+# the manifest's own directory, so the manifest stays valid if the whole
+# dataset tree is moved or copied elsewhere intact.
+MANIFEST_SUFFIX = ".manifest"
+
+
+def _read_manifest(path: Path) -> list[Path]:
+    files = []
+    for line in path.read_text().splitlines():
+        line = line.strip()
+        if not line or line.startswith("#"):
+            continue
+        resolved = (path.parent / line).resolve()
+        if not resolved.is_file():
+            raise FileNotFoundError(f"{path} lists missing file: {resolved}")
+        files.append(resolved)
+    if not files:
+        raise FileNotFoundError(f"manifest {path} lists no files")
+    return files
+
 
 def find_parquet_files(path: str | Path) -> list[Path]:
     p = Path(path)
+    if p.suffix == MANIFEST_SUFFIX:
+        return _read_manifest(p)
     if p.is_dir():
         files = sorted(p.glob("*.parquet"))
         if not files: