Add SP2-XR I/O support (HK, sp2b binary, PbP CSV)

[BarbaraBertozzi]

Summary

Adds I/O support for the SP2-XR (Single Particle Soot Photometer Extended
Range) instrument variant.

What's included

• pysp2.io.read_sp2xr_hk_file — housekeeping reader (CSV/ZIP), excluding
  firmware-computed PSD bin columns
• pysp2.io.read_sp2xr_hk_psd — accesses the firmware-computed PSD
  histograms as a 2D dataset shaped (time, num_bins)
• pysp2.io.read_sp2xr — binary .sp2b reader
• pysp2.io.read_sp2xr_pbp — Particle-by-Particle CSV/ZIP reader with
  auto-detection of the matching HK file for absolute datetimes. Firmware-
  calibrated Scatter Size (nm) and Incand Mass (fg) dropped by default to
  encourage recalibration with consistent curves
• Calibration provenance: scattering and incandescence calibration CSVs
  (matching *_Scatt_*.csv / *_Incan_*.csv) are auto-located and attached
  as dataset attributes wherever firmware-calibrated values are kept
• Example scripts (plot_read_sp2xr.py, plot_hk_sp2xr.py)
• pytest coverage for the four readers
• Small SP2-XR sample dataset under pysp2/testing/data/SP2XR/

[rcjackson]

Examples, unit tests, and docs are all here! We welcome this new contribution @BarbaraBertozzi!