Add tests for truncation feature

[pjt222]

Context

The truncation feature was added in commits 338da13 and a611b2e but has no test coverage.

Files affected:

• R/spiral_math.R - truncate_spiral_points() function
• R/ui_controls.R - truncation UI controls

Test Cases Needed

1. Basic functionality

   • Truncation with factor=2.0 (default)
   • Truncation with factor=1.0 (aggressive, ~50% points)
   • Truncation with factor=0.1 (minimum allowed)

2. Edge cases

   • Factor < 0.1 should error
   • Factor that removes all but minimum points
   • Empty input handling
   • Single point input

3. Integration with Voronoi

   • Verify truncated points produce valid Voronoi diagrams
   • Cache keys include truncation parameters

Related

• Part of Issue 🧪 Expand Test Coverage #4 (Expand Test Coverage)
• Depends on recent commits a611b2e, 338da13

Priority: High