Add ProcessRouter for physics-process-based expert gating

Routes on the physics process (Compton, phot, brems, ...) that ends a
step, supervised by a small classifier since process is a post-step
outcome unobservable at gate time. Threads a process label end-to-end
through the data pipeline (loader, build_features, dataset batches,
training loss/checkpointing) alongside the existing EnergyRouter.

tests/test_loader.py

@@ -1,6 +1,7 @@
+import pandas as pd
 import pytest
 
-from giant.data.loader import find_parquet_files
+from giant.data.loader import build_process_map_from_files, find_parquet_files
 
 
 def _touch(path):
@@ -58,3 +59,36 @@ def test_manifest_with_no_entries_raises(tmp_path):
 
     with pytest.raises(FileNotFoundError):
         find_parquet_files(manifest)
+
+
+def test_build_process_map_from_files_keeps_most_frequent(tmp_path):
+    """process counts: eIoni=5, phot=3, compt=2, Rayl=1 — with n_experts=3, only
+    the top 2 (eIoni, phot) get their own index; compt/Rayl share the "other"
+    (last) index."""
+    process = (
+        ["eIoni"] * 5 + ["phot"] * 3 + ["compt"] * 2 + ["Rayl"] * 1
+    )
+    path = tmp_path / "shard-000.parquet"
+    pd.DataFrame({"process": process}).to_parquet(path)
+
+    proc_map = build_process_map_from_files([path], n_experts=3)
+
+    assert proc_map["eIoni"] == 0
+    assert proc_map["phot"] == 1
+    assert proc_map["compt"] == 2
+    assert proc_map["Rayl"] == 2
+    assert set(proc_map.values()) <= {0, 1, 2}
+
+
+def test_build_process_map_from_files_spans_multiple_files(tmp_path):
+    path_a = tmp_path / "a.parquet"
+    path_b = tmp_path / "b.parquet"
+    pd.DataFrame({"process": ["eIoni"] * 3 + ["phot"] * 1}).to_parquet(path_a)
+    pd.DataFrame({"process": ["phot"] * 4 + ["compt"] * 1}).to_parquet(path_b)
+
+    # phot: 1+4=5 total > eIoni: 3 > compt: 1
+    proc_map = build_process_map_from_files([path_a, path_b], n_experts=3)
+
+    assert proc_map["phot"] == 0
+    assert proc_map["eIoni"] == 1
+    assert proc_map["compt"] == 2