Add extensive unit test coverage for curveUtils math/geometry functions

## Background

The `curveUtils.ts` module contains pure functions performing complex math and geometry calculations:
- `createMonotoneInterpolator`: monotone cubic Hermite interpolation
- `histogramToPath`: SVG path generation from histogram data
- `curvesToLUT`: lookup table generation from curve points

Currently, basic test coverage exists in `curveUtils.test.ts`, but as a manually-coded math/geometry engine, we need significantly more extensive coverage.

## Objective

Expand the test suite to cover:
- All edge cases for each function
- The many permutations of inputs and boundary conditions
- Deterministic assertions for all calculated results
- Numerical stability and precision edge cases
- Invalid/extreme input handling

Since these are pure functions with deterministic outputs, they are ideal candidates for comprehensive testing.

## Context

Requested by: @christian-byrne
Related PR: #8860
Comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/8860#discussion_r2838847344

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