PR: SVG sprite parser selection via pragma

Title

Enable SVG sprite parser selection via pragma (Nano default)

Description

This PR implements pragma-based selection for SVG sprite parsing while keeping the Nano parser as the default to preserve backward compatibility. The SAX parser can be enabled explicitly with a pragma and remains opt-in. TeaVM builds always force Nano to avoid missing javax.xml.parsers during JavaScript generation.

Motivation / Context

Maintainers requested that Nano remain the default parser and SAX be explicitly enabled. This change provides a simple, diagram-local switch that avoids breaking existing users while allowing testing and adoption of the SAX parser.

Related PR: https://github.com/plantuml/plantuml/pull/2548 (closed, #2548)

Current behavior

• SVG sprite parsing uses Nano by default.
• There was no pragma-based selector for SAX vs Nano.

New behavior

• Nano remains the default.
• The SAX parser is enabled per diagram using pragmas:

!pragma svgparser sax

or

!pragma svgparser nano

TeaVM behavior

TeaVM does not include javax.xml.parsers. The parser factory forces Nano when TeaVM.isTeaVM() is true. TeaVM resolves this branch at compile time, so the JVM-only SAX path is removed from generated JavaScript output and class stubs.

Tests

• Updated SVG sprite tests to use the SAX pragma where appropriate.
• Parser selection tests now validate pragma behavior and Nano default.

Documentation

• Documented Nano vs SAX constraints in the svg parser package Javadoc.
• Added a package README that highlights Nano default behavior and SAX capabilities/constraints.

Notes / Limitations

• SAX parser supports inline attributes only (no CSS <style> blocks or class selectors).
• Gradient rendering is simplified (first/last stops only; direction approximated).
• Embedded raster images are supported only via data URIs (PNG/JPEG).

SAX Advantages

• Broader SVG element coverage: rect, circle, ellipse, line, polyline, polygon, path, text, image, group, defs, symbol, use.
• Transform parsing (translate, rotate, scale, matrix).
• Consistent inline styling across supported elements.
• Linear gradient fills (with simplified stop handling).

Files changed (high level)

• Pragma key and selection logic for SVG parser choice.
• TeaVM fallback to Nano.
• Tests and SVG sprite fixtures updated to use !pragma svgparser sax.
• SVG parser documentation updates.