fix: show height above lowest point in SimplePendulum info panel

[TechGenius-Karan]

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🔍 Description

What was wrong

The Height field in the Simple Pendulum info panel was always showing a large negative number (e.g. -2.4 m), even when the pendulum was just hanging still. This happened because the code was using -position.y to
calculate height.

The problem: the simulation uses a Y-up coordinate system where the anchor point sits at a large positive Y
value (90% of the canvas height in meters). So negating the bob's Y position doesn't give a meaningful height —
it gives a big negative number that doesn't relate to the pendulum's swing at all.

What the fix does

Height is now calculated as the bob's height above its lowest point (the equilibrium position at the bottom
of the swing):

height = position.y - (anchor.y - length)

• At rest (0° angle) → 0.00 m ✓
• As the bob swings up → height increases correctly in metres ✓

Closes #5 (if applicable)

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• Followed the CONTRIBUTING.md guidelines

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📂 Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📝 Documentation update
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• 🎨 UI/UX enhancement
• 🔒 Security improvement

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