Feat/line graph smoothing

[nebojsa-peric]

LineGraph: monotone smoothing mode (smooth="monotone")

Bezier smoothing (smooth={true}) overshoots the data range on steep slopes —
curves dip below zero on non-negative series or bulge past the actual maximum,
making values appear that don't exist in the data.

This PR adds smooth="monotone": monotone cubic interpolation (Fritsch–Carlson,
same algorithm as d3's curveMonotoneX). Tangents are clamped at local extrema,
so the curve is guaranteed to stay within the vertical bounds of the data. Works
for both line and area rendering.

• smooth={true} is unchanged; smoothingRatio applies to bezier only.
• New litmus (features/charts/line-graph/SmoothingOvershoot.js) compares both
  modes side by side on datasets engineered to trigger overshoot.
• Verified numerically: sampled curves stay within [dataMin, dataMax] on all
  litmus datasets and edge cases (two-point spans, duplicate x).

[nebojsa-peric]

Proposed changelog & breaking-changes entries

Since smooth now always renders monotone curves (bezier smoothing removed, smoothingRatio deprecated and ignored), this likely warrants a changelog entry with a breaking-changes note. Proposal below — the version number is a placeholder, adjust to whatever the release ends up being.

homedocs/src/pages/changelog.mdx:

## cx\@26.8.0

**Breaking Changes**

- `LineGraph` smooth line rendering now uses monotone cubic interpolation instead of cubic Bézier curves. The smoothed curve never overshoots the vertical range of the data, so graphs no longer suggest values that don't exist — most noticeable around spikes and steep slopes next to flat segments. Existing charts using `smooth` will render slightly differently ([#1312](https://github.com/codaxy/cxjs/pull/1312))
- `smoothingRatio` on `LineGraph` is deprecated and ignored — monotone interpolation has no configurable curvature. It is safe to remove it from existing code. See [Breaking Changes](/docs/intro/breaking-changes#2680---linegraph-smoothing) for details

homedocs/src/pages/docs/intro/breaking-changes.mdx (new section above 26.3.1):

## 26.8.0 - LineGraph Smoothing

`LineGraph` smooth line rendering (`smooth={true}`) now uses monotone cubic interpolation instead of cubic Bézier curves.

### Why This Change?

Bézier-based smoothing overshoots the actual data range on steep slopes next to flat segments (steps, spikes, zeros),
making the graph suggest values that don't exist in the data — e.g. a smoothed line dipping below zero next to a spike.
Monotone cubic interpolation keeps the curve within the vertical bounds of the data, so smoothed charts remain truthful.

### What To Expect

- No code changes are required — `smooth` keeps working as before.
- Smoothed lines and areas render slightly differently: tighter around spikes and steep slopes, with no dips or bumps
  beyond the actual data range.
- `smoothingRatio` is deprecated and ignored, as monotone interpolation has no configurable curvature.
  It is safe to remove it from existing code.