v6.1.0

Added

• The flpq-check-archive console command (installed with the package):
  validates a graph archive's structure, so contributors check their
  archives with pip install flpq-data alone — no script download.
• utils/add_reachable_pairs.py: derives the reachable-pairs rows of one
  validated archive (one row per query directory, with the declared entry
  count of its results.mtx as num_reachable_pairs) and inserts them into
  flpq_data/dataset/reachable_pairs.csv, preserving the file's sort order
  and refusing duplicate rows and category disagreements.

Changed

• The archive structure checker moved from utils/check_archive_structure.py
  to flpq_data/tools/ (console command flpq-check-archive); the S3 client
  factory and bucket constants moved to flpq_data.dataset.s3.

• Data contributions are issue-first: a single "CONTRIBUTE NEW DATA" issue
  carries one or more validated archives (one graph each), the per-archive
  validation output and SHA256, and the motivation; maintainers upload and
  merge the data into the dataset. The Graphs page documents the flow, the
  four contribution cases, and the consistency invariants — no graph without
  a query, no query without reference results, no item without description
  and motivation.

Removed

• The ADD NEW GRAPH / ADD NEW GRAMMAR issue templates and the
  new_graph / new_grammar PR templates: data has no PR path, and a
  grammar is accepted only together with the archive that instantiates it as
  a query with reference results. A generic PR template covers code
  contributions.