Fix division by zero and array bounds violations

[Copilot]

Systematic scan identified 6 critical safety issues causing undefined behavior or crashes with edge-case inputs.

Fixes

LatheGeometry: Division by (points.size() - 1) at line 49

• Added points.size() >= 2 guard, early return if insufficient

AssimpLoader: Bone weight normalization divides by sum without null check

• Guard with sum > 0.0f, fallback to uniform distribution (0.25 each) for degenerate case
• Documents this as compatibility fallback for malformed models

Lut colormap: Three issues in color lookup table

• Guard n <= 0 by clamping to 1 (maintains backward compatibility)
• Protect (max - min) division with zero check
• Clamp colorPosition to [0, n-1] preventing out-of-bounds at boundaries

SplineCurve: Boundary check allows points[intPoint + 2] out-of-bounds

• Changed condition from > size - 3 to >= size - 2
• Added empty points guard

SVGFunctions: Division by ay without null check

• Added ay != 0 check, early return BEHIND for degenerate case (both ax, ay zero)

All changes are minimal guards - no functional behavior modified for valid inputs. All 28 existing tests pass.

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