Optimize curvesToLUT: Replace binary search with monotonic segment walk

## Context
In PR #8860, the `curvesToLUT` function currently calls `createMonotoneInterpolator` which performs a binary search for each of the 256 samples to locate the correct curve segment.

## Optimization Opportunity
Since `x = i / 255` is strictly increasing in the sampling loop, we can walk through the curve segments linearly instead of performing a binary search for every sample. This would improve the time complexity from O(256 log n) to O(256 + n).

## Proposed Approach
- Extract the interpolation logic (xs, ys, slopes) into a reusable precomputation function
- In `curvesToLUT`, maintain `lo` and `hi` segment indices across iterations
- Advance the segment pointers monotonically as x increases
- Inline the Hermite interpolation formula

## References
- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/8860
- Discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/8860#discussion_r2838856892
- Requested by: @christian-byrne

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