Optimize histogramToPath performance in curveUtils

## Performance Optimizations for histogramToPath

Requested by @christian-byrne in [PR #8860 review comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/8860#discussion_r2838865130).

### Suggested Optimizations

The `histogramToPath` function in `src/components/curve/curveUtils.ts` has several opportunities for performance improvements:

1. **Use Array.join instead of string concatenation** - Building the SVG path string using an array and joining at the end is more efficient than repeated string concatenation in the loop.

2. **Replace toFixed with manual rounding** - Manual rounding (`Math.round(x * 10000) / 10000`) is faster than `toFixed(4)` in hot paths.

3. **Hoist step calculations and use accumulation** - Instead of computing `i * step` on each iteration, accumulate `x += step` to reduce multiplication operations.

See the [original comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/8860#discussion_r2838865130) for detailed code examples.

### Context

- Related PR: #8860
- File: `src/components/curve/curveUtils.ts`
- Function: `histogramToPath`

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_This issue was created as a follow-up to optimize the CurveEditor component introduced in #8860._

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