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/merge_layers.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+#
+# Collapse all accumulated layers in the final parquet datasets into a
+# single clean layer. Use this after several incremental adds via
+# add_months.sh when you want to reduce the number of partition files.
+#
+# Reads the existing by_subreddit / by_author datasets, re-sorts everything,
+# writes to temp paths, then atomically replaces the originals via rename.
+# The old directories are removed once the new ones are in place.
+#
+# If the process is interrupted after writing the .merging directories but
+# before the renames complete, re-run — the .merging directories will be
+# overwritten and the originals are still intact. If interrupted after the
+# renames, the .old directories are left behind; delete them manually once
+# satisfied with the output.
+#
+# To add new months without merging, use add_months.sh.
+# To rebuild everything from raw dumps, use build_from_scratch.sh.
+#
+# NOTE: This script and its workflow are written but not yet tested.
+# Remove this notice after a successful end-to-end run.
+#
+# Every command below is independently runnable for debugging.
+
+set -e
+cd "$(dirname "$0")"
+
+# merge and collapse comments layers
+start_spark_and_run.sh 1 comments_merge.py
+
+# merge and collapse submissions layers
+start_spark_and_run.sh 1 submissions_merge.py