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/add_new_month.sh

@@ -34,15 +34,15 @@ fi
 # --- Part 1: parse the new month's dumps (no wipe) -------------------------
 
 # parse the new comments file
-python3 parquet_part1.py comments parse_dump "RC_${MONTH}.zst"
+python3 comments_part1.py parse_dump "RC_${MONTH}.zst"
 
 # parse the new submissions file
-python3 parquet_part1.py submissions parse_dump "RS_${MONTH}.zst"
+python3 submissions_part1.py parse_dump "RS_${MONTH}.zst"
 
 # --- Part 2: re-sort the full corpus including the new data ---------------
 
 # sort comments and overwrite reddit_comments_by_{subreddit,author}.parquet
-start_spark_and_run.sh 1 parquet_part2.py comments
+start_spark_and_run.sh 1 comments_part2.py
 
 # sort submissions and overwrite reddit_submissions_by_{subreddit,author}.parquet
-start_spark_and_run.sh 1 parquet_part2.py submissions
+start_spark_and_run.sh 1 submissions_part2.py