PR: SVG Sprite Linear Gradient Multi-Stop + SpreadMethod

Summary

This PR expands SVG sprite rendering to preserve multi-stop linear gradients and honours SVG spreadMethod values across SVG and PNG outputs.

Key changes

• SAX sprite parser now captures linearGradient multi-stop definitions and spreadMethod.
• Gradient model extended to carry spread method for repeat/reflect behavior.
• SVG output emits multi-stop <linearGradient> with optional spreadMethod.
• PNG (G2D) output maps spreadMethod to LinearGradientPaint cycle methods.
• Added visual verification sprite fixtures for pad/reflect/repeat.
• Added a regression test for standard (non-sprite) gradient vectors in SVG output.

Files touched (high level)

• Parser: src/main/java/net/sourceforge/plantuml/svg/parser/SvgSaxParser.java
• New HColor Gradient model: src/main/java/net/sourceforge/plantuml/klimt/color/HColorLinearGradient.java
• SVG renderer: src/main/java/net/sourceforge/plantuml/klimt/drawing/svg/SvgGraphics.java
• PNG renderer: src/main/java/net/sourceforge/plantuml/klimt/drawing/g2d/DriverRectangleG2d.java
• SVG path fill handling: src/main/java/net/sourceforge/plantuml/klimt/drawing/svg/DriverPathSvg.java
• Test samples, Fixtures: src/test/resources/svg-sprites/linearGradientSpreadPad.puml, src/test/resources/svg-sprites/linearGradientSpreadReflect.puml, src/test/resources/svg-sprites/linearGradientSpreadRepeat.puml
• Tests: src/test/java/net/sourceforge/plantuml/svg/parser/SvgSpriteImageGenerationTest.java, src/test/java/net/sourceforge/plantuml/svg/StandardGradientSvgTest.java

Tests

• ./gradlew test --tests net.sourceforge.plantuml.svg.parser.SvgSpriteImageGenerationTest
• ./gradlew test --tests net.sourceforge.plantuml.svg.StandardGradientSvgTest

Notes

• spreadMethod is preserved in SVG and mapped to PNG cycle methods; default remains pad when unspecified.
• Visual fixtures allow manual verification of pad/reflect/repeat behavior.
• Standard gradient regression coverage: StandardGradientSvgTest renders a simple diagram (non-sprite) and asserts SVG gradient vectors for |, -, /, and \\ policies.
• Design note: gradient stop offsets are normalised to a strictly increasing sequence (ties are nudged by a tiny epsilon) to satisfy Java2D LinearGradientPaint requirements while preserving SVG semantics of non-decreasing offsets.

Why non-parser packages changed

• G2D rendering paths were updated so PNG output matches SVG behaviour. The SVG sprite parser produces an HColorLinearGradient, and the G2D drivers (via src/main/java/net/sourceforge/plantuml/klimt/drawing/g2d/DriverRectangleG2d.java) must translate this into LinearGradientPaint with the correct cycle method; otherwise spreadMethod would be silently ignored in PNG.
• src/main/java/net/sourceforge/plantuml/klimt/color/HColors.java was extended to handle the new HColorLinearGradient in noGradient() so callers that intentionally strip gradients continue to behave consistently when the new gradient type appears.
• src/main/java/net/sourceforge/plantuml/klimt/drawing/svg/DriverPathSvg.java avoids flattening gradients when stroke and fill are the same, which previously dropped multi-stop data in SVG output.

The G2D change is intentionally small and scoped to recognising the new gradient type alongside the existing two-stop HColorGradient. Typical call sites look like this in src/main/java/net/sourceforge/plantuml/klimt/drawing/g2d/DriverPathG2d.java:

if (back instanceof HColorGradient || back instanceof HColorLinearGradient) {
	final Paint paint = DriverRectangleG2d.getPaintGradient(..., back);
	g2d.setPaint(paint);
	g2d.fill(path);
}

Regression risk scope

• Changes are limited to rendering of linear gradients in SVG sprites and to generic gradient flattening behaviour for the new gradient type.
• Non-gradient rendering paths are unchanged; tests focus on the sprite pipeline but a regression test for standard gradients has been added.