datasets/: replace add_new_month with layered append workflow

Add add_months.sh and merge_layers.sh implementing a layered append
strategy for incremental dataset updates. Each incremental run appends
new sorted partition files alongside existing ones rather than re-sorting
the full corpus, which is prohibitively slow at this dataset scale.

- dumps_helper.py: sort_and_write gains indir/mode params; new
  merge_layers function collapses accumulated layers via atomic rename
- comments_part2.py, submissions_part2.py: expose --indir/--mode via Fire
- add_months.sh: new layered append script (not yet tested)
- merge_layers.sh: new layer collapse script (not yet tested)
- comments_merge.py, submissions_merge.py: Spark entry points for merge
- add_new_month.sh: deleted (full re-sort each add is redundant with
  build_from_scratch at corpus scale)
- README.md: document three workflows; flag untested sections

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

datasets/comments_merge.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python3
+"""Collapse all layers in the comments final datasets into a single clean layer.
+
+Must be launched from a login node via the Hyak-provided wrapper:
+  start_spark_and_run.sh 1 comments_merge.py
+
+See merge_layers.sh and dumps_helper.merge_layers for details.
+"""
+
+from dumps_helper import COMMENTS, merge_layers
+
+
+if __name__ == "__main__":
+    merge_layers(COMMENTS)