HeatStressR 2.1.0 — Multi-core support; prep for CRAN

Added

• wbgt.Liljegren() batch execution accepts an explicit workers count.
  Values above one use cross-platform PSOCK workers while retaining output,
  warning, and diagnostic ordering. Worker count is user-selected within the
  currently permitted worker limit; it is not automatically reduced based on
  workload size except that it is capped at the number of input rows.
• New internal parallel execution layer (R/wbgt-parallel.R) providing
  max_liljegren_workers(), validate_workers(), split_liljegren_chunks(),
  solve_liljegren_batch_raw_chunk(), and solve_liljegren_parallel().
• Benchmark harnesses for multicore performance evaluation:
  benchmarks/benchmark-liljegren-workers-1-to-6x87600.R (sweeping worker
  counts with configurable ROWS_PER_WORKER) and
  benchmarks/benchmark-liljegren-parallel.R (annual workload
  profiling). Results recorded on Apple M2 Max.
• DESCRIPTION now lists parallel as an explicit import.

CRAN preparation

• Standardised licence metadata and package authorship.
• Added package and Liljegren-model citations.
• Added runnable Liljegren examples and expanded installed documentation.
• Clarified that HeatStressR is an R implementation of the Liljegren method,
  not a claim of improvement over the original Liljegren program.
• Made worker-limit detection respect CRAN check core limits.
• Added CRAN submission notes and related regression tests.

Changed

• wbgt.Liljegren() with engine = "batch" automatically falls back to
  multi-worker PSOCK parallel execution when workers > 1, splitting rows
  across chunks and combining results with preserved diagnostic attributes.
• Worker count validation enforces finite integer input and caps at the
  currently permitted worker count; no workload-size heuristic is applied
  beyond the row-count cap.

Fixed

• Batch worker count is now capped at the detected logical CPU limit to
  prevent oversubscription on multi-socket or hyperthreaded systems.