Implement ITU-R P.618-14 Earth-space propagation recommendation

[inigodelportillo]

Summary

• Adds _ITU618_14 class delegating to _ITU618_13; the core algorithms (rain attenuation, rain probability, XPD, scintillation el≥5°, total attenuation) are identical between the two revisions — P.618-14 extends the §2.4.1 scintillation validity range from 20 GHz to 55 GHz without changing the equations
• Updates the default active version in the _ITU618 wrapper from 13 to 14
• Fixes the scintillation_attenuation and scintillation_attenuation_sigma docstrings to say "55 GHz" (was stale "20 GHz" inherited from P.618-12)
• Adds ITUR618_14TestCase to both ITU_validation_test.py and ITU_validation_report_test.py
• Adds four validation CSV files extracted from the ITU Validation examples spreadsheet Rev 8.3.0 (not Rev 5.1 used for P.618-13): A_rain, A_sci, A_total, A_xpd
  • A_sci: 48 rows — P.618-14 drops the out-of-range p = 0.001% cases (valid range is 0.01 ≤ p ≤ 50%)
  • A_total: 48 rows with new frequencies (13.75, 14.25, 17, 20 GHz) and locations vs the P.618-13 set
• Adds docs/validation/itur618_14.rst documentation page

Test plan

• ITUR618_14TestCase — 6 unit tests in ITU_validation_test.py all pass
• ITUR618_14TestCase — 5 report tests in ITU_validation_report_test.py all pass (generates correct HTML tables from Rev 8.3.0 data)
• ITUR618_13TestCase tests unaffected

🤖 Generated with Claude Code

[inigodelportillo]

This should link to the right file, the new version.

[inigodelportillo]

The HTML file also need to be included as part of this PR.