Drop orphaned child tracks instead of nulling secondary targets

A listed child_track_id can fail to match any first-step row (e.g. a
secondary absorbed below the tracking threshold at birth). The
parent->child left join in _add_secondary_attributes left these as
nulls, which silently became NaN once the parquet round-tripped
through the loader's float32 padding — poisoning every later secondary
slot in that step via the cumulative "remaining budget" in
encode_secondaries, while e_sec quietly undercounted and n_sec (from
len(child_track_ids)) overcounted relative to the actual lists.

Drop orphans from both the per-secondary lists and child_track_ids
itself so downstream counts stay consistent, and thread the per-file
orphaned count back through convert_steps_to_parquet so both the
sequential and --jobs>1 batch paths in `dwarf convert` can report an
aggregate total instead of relying on grepping printed output.

Also floors encode_secondaries' slot-0 budget to _EPS (matching the
i>0 branch), fixing a harmless but noisy 0/0 divide warning on
zero-secondary steps.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/dwarf.py

@@ -116,14 +116,21 @@ def convert(
                 "error: --output can only be used with a single input file", err=True
             )
             raise typer.Exit(1)
+        total_orphaned = 0
         for root_file in root_files:
-            convert_steps_to_parquet(
+            _, n_orphaned = convert_steps_to_parquet(
                 root_file,
                 output_path=output,
                 batch_size=batch_size,
                 tree_name=tree,
                 compression=compression_value,
             )
+            total_orphaned += n_orphaned
+        if total_orphaned:
+            typer.echo(
+                f"\n{total_orphaned} orphaned child track(s) dropped across "
+                f"{len(root_files)} file(s)."
+            )
         return
 
     if output is not None: