End-to-End Parallel Liljegren WBGT

• Multi-worker wbgt.Liljegren() execution now uses a temporary
  foreach/doParallel PSOCK backend. Each worker calculates solar geometry
  and completes its WBGT shard end-to-end; the caller's existing foreach
  backend is restored after the call.
• Parallel benchmark runners now compare a fixed total row count across worker
  counts. The worker sweep accepts LILJEGREN_PARALLEL_ROWS (default
  1,000,000) rather than increasing rows per worker.
• Reduced the benchmark suite to the maintained three-way pre-fork comparison
  and fixed-workload internal-worker sweep, with refreshed result records.