datasets/: split parquet scripts; share logic in dumps_helper.py

Follows the helper-module pattern used in similarities/. Replaces
parquet_part1.py and parquet_part2.py (the merged single-file versions
from the previous commit) with:

- dumps_helper.py — schemas, simdjson parser, a generic parse_record
  loop with per-field handler dispatch, and parse_dump / gen_task_list
  / sort_and_write workers. The only per-type code is the field-handler
  dicts and the type-config dicts (COMMENTS, SUBMISSIONS) at the top.
- comments_part1.py, submissions_part1.py — thin Part 1 entry points
  with fire CLIs (parse_dump, gen_task_list).
- comments_part2.py, submissions_part2.py — thin Part 2 entry points
  for the Spark sort. pyspark is imported lazily inside sort_and_write
  so Part 1 callers don't pay the import cost.

Unifies on simdjson for both types (drops the json import), which is
faster on the comments dumps. Field-handler dicts make adding a new
type or field a one-place edit.

Also fixes a latent bug in the original: the FIELDS lists didn't
include time_edited (only the schema did), so error-path rows were
short by one element vs. the schema and would have failed pandas /
pyarrow alignment for any row that hit a JSON parse error. The new
FIELDS lists match the schemas exactly, and the _edited handler
returns a (edited, time_edited) tuple that the generic parse loop
expands.

Runners and README updated for the new CLIs.

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

datasets/comments_part1.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python3
+"""Part 1 for comments: parse one RC_*.zst dump into a parquet file.
+
+CLI:
+  comments_part1.py parse_dump RC_2018-08.zst
+  comments_part1.py gen_task_list
+  comments_part1.py parse_dump RC_2018-08.zst --dumpdir=/tmp/in --outdir=/tmp/out
+"""
+
+import fire
+from dumps_helper import COMMENTS, parse_dump, gen_task_list
+
+
+def _parse_dump(partition, dumpdir=None, outdir=None):
+    parse_dump(COMMENTS, partition, dumpdir=dumpdir, outdir=outdir)
+
+
+def _gen_task_list(dumpdir=None, tasklist=None):
+    gen_task_list(COMMENTS, 'comments_part1.py', dumpdir=dumpdir, tasklist=tasklist)
+
+
+if __name__ == "__main__":
+    fire.Fire({'parse_dump': _parse_dump,
+               'gen_task_list': _gen_task_list})