fix: correct sidereal_day magnitude and standardize digit separators in astronomy.h

[Copilot]

sidereal_day evaluated to ~861.6 s (~14 min) instead of the correct ~86164 s (~23h 56m 4s) due to a misplaced digit separator producing a numerator 100× too large relative to the denominator. Two other constants in the same file had non-standard digit groupings.

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
• 🔧 Build system / CI changes
• 🧪 Test improvements
• ♻️ Code refactoring
• 🎨 Code style improvements

🔗 Related Issues

Closes #786

📝 Changes Made

• sidereal_day: mag_ratio<86'164'0905, 1'000'000> → mag_ratio<861'640'905, 10'000>
  (86'164'0905 is a 9-digit integer; dividing by 10⁶ yielded 861.6 s, not 86164 s)
• Julian_year: mag_ratio<365'25, 100> → mag_ratio<36'525, 100>
  (non-standard 2-digit terminal group obscured the actual integer value 36525)
• tropical_year: mag_ratio<365'242189, 1'000'000> → mag_ratio<365'242'189, 1'000'000>
  (6-digit terminal group split into standard 3-digit groups)

All values are mathematically correct after the changes; sidereal_day was the only functional bug.

🌟 Additional Context

Non-standard digit separator groupings (e.g. 365'25, 365'242189) can silently misrepresent the scale of a literal at a glance — the same class of issue that caused the sidereal_day bug.

✅ Checklist

• I have read the Contributing Guidelines
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have updated the project's documentation to cover the modified or new functionality
  (if this change affects user-facing features)

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