Fix critical reliability prediction bug - required_snr parameter issue

[skyelaird]

PROBLEM:
DVOACAP was predicting 0-1.2% reliability when VOACAP showed 20-100% reliability, making it completely unusable for practical propagation predictions. Strong 40+ dB SNR signals were marked as "unlikely".

ROOT CAUSE:
The required_snr parameter was using VOACAP's validation test value (73 dB) instead of a realistic communications threshold (10 dB for SSB). This caused the reliability calculation to treat even strong signals as insufficient.

SOLUTION:

• Set required_snr = 10.0 in compare_voacap_charts.py for realistic predictions
• Keep default at 73.0 in PredictionParams for validation compatibility
• Document the parameter meaning and recommended values by mode

RESULTS:
Before fix: 612km path showed 1.2% reliability (VOACAP: 80%) After fix: 612km path shows 93.5% reliability (VOACAP: 80%)

Reliability predictions are now realistic and physically sensible:

• Strong SNR (40+ dB) → High reliability (93-98%)
• Moderate SNR (28-30 dB) → Good reliability (85-86%)

This makes DVOACAP suitable for operational HF propagation prediction.

See REQUIRED_SNR_PARAMETER_FIX.md for complete analysis.

Fixes tool propagation issue identified in VOACAP cap wheel comparison.

Description

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Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Code refactoring
• Test coverage improvement

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python test_voacap_reference.py

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• Code follows PEP 8 style guidelines
• Docstrings added/updated for public functions
• Type hints added where appropriate
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• Documentation updated (README, USAGE, etc.)
• CHANGELOG.md updated
• No new warnings or errors introduced

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• Code quality is acceptable
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• No security issues introduced
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