Fix trailing-newline bug: RhoData treats blank lines as sample IDs

`RhoData.member_list` includes empty strings from trailing newlines in
filelists, causing `FileNotFoundError: .CHGCAR`. Write filelist without
trailing newline. Always use temp data root (even for all samples) to
ensure clean filelist.

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

Author: ryan-williams

modal/benchmark.py

@@ -110,14 +110,19 @@ def run_benchmark(
         samples = len(subset)
         log.info("Using all %d eligible samples", samples)
 
-    # Create temp data root with symlinks + custom filelist
+    if not subset:
+        raise ValueError(
+            f"No eligible samples (total={len(all_ids)}, max_file_size={max_file_size}MB)"
+        )
+
+    # Create temp data root with symlinks + clean filelist (no trailing newline)
     data_root = "/tmp/benchmark_data"
     Path(data_root).mkdir(parents=True, exist_ok=True)
     for subdir in ["data", "label"]:
         link = Path(data_root) / subdir
         if not link.exists():
             link.symlink_to(Path(DATA_ROOT) / subdir)
-    Path(data_root, "mp_filelist.txt").write_text("\n".join(subset) + "\n")
+    Path(data_root, "mp_filelist.txt").write_text("\n".join(subset))
 
     log.info(
         "Benchmark: gpu=%s, epochs=%d, channels=%d, blocks=%d, samples=%d",