Feature Request: Path-Aligned Gradient

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### Problem

Current gradient rendering is defined in screen/user space and is not aware of path geometry. As a result, it cannot produce a continuous color transition that follows a polyline or curved path.

### Desired Behavior

We want to support gradients that are bound to the path itself, such that:

* Color transitions follow the direction and length of a stroke or path
* The gradient is continuous across multi-segment polylines and curves
* No visible discontinuities appear at segment joins or sharp turns
* The gradient remains consistent under transforms (scale, rotate, etc.)

### Goal

Enable a single gradient definition to smoothly map along the entire path as if the color is driven by the path’s intrinsic parameterization (e.g. arc-length), rather than by screen-space coordinates.

Start by tracing the current gradient rendering path and how paths, polylines, curves, and transforms are represented. Determine where screen/user-space gradients are evaluated and how an intrinsic path parameterization could span segment joins. Done means a single gradient produces a continuous, transform-consistent transition across complete paths without visible discontinuities.

Written by the indexing model from the issue text.