Add SVG mix-blend-mode support + fix gradient rendering

[rcoenen]

Summary

Add SVG mix-blend-mode support and fix gradient rendering when exporting SVG → PDF.

Before (PDF render)	After (PDF render)

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Closes #139.

• SVG mix-blend-mode support: Parse the CSS property from SVG style attributes and map all 16 CSS blend modes to their PDF /BM operator equivalents via the existing getAlpha() infrastructure.
• Gradient stop color normalization: Named SVG colors (white, red, etc.) resolve to CMYK while hex colors (#FFFFFF, #00C147) resolve to RGB. When mixed in gradient stops, the PDF gradient breaks silently. All stop colors are now normalized to hex RGB.
• Gradient forward reference pre-scan: Figma (and other tools) place <defs> at the end of the SVG. The single-pass parser couldn't resolve forward gradient references. A lightweight prescanSVGGradients() pre-parse now collects all gradient definitions before the main render pass.
• Blend modes work on gradient fills: Moved blend mode detection before the gradient early return in parseSVGStyleFill() so /BM applies to gradient-filled elements too.

Test Input

Original SVG used for reproduction (exported from Figma): BlendModeTest.svg

Changes

• src/SVG.php — all source changes (blend mode parsing, stop color fix, prescan method)
• test/SvgBlendModeTest.php — 26 integration tests covering all blend modes, inheritance, gradient combinations
• test/visual/BlendModeTest.svg + test/visual/blend-mode-smoke.php — visual smoke test with real Figma export

Test plan

• All 26 unit/integration tests pass (vendor/bin/phpunit --no-coverage)
• Visual smoke PDF matches reference SVG (blend modes + gradient rendering)
• Unmodified Figma SVG export renders correctly (forward refs + named colors)

🤖 Generated with Claude Code

[CLAassistant]

All committers have signed the CLA.

[rcoenen]

Follow-up fix pushed: preserves effective SVG opacity when mix-blend-mode is enabled (previously repeated ExtGState writes could overwrite opacity / fill-opacity / stroke-opacity).

Also added regression tests for blend mode + opacity combinations and cleaned up prescanSVGGradients() callback typing/naming (PHPMD/PHPStan clean on touched files).

Validated locally:

• phpunit test/SvgBlendModeTest.php ✅ (28 tests, 36 assertions)
• targeted PHPStan on src/SVG.php + test/SvgBlendModeTest.php ✅
• visual smoke test regenerated ✅

[nicolaasuni]

Thank you for your contribution.
I manually merged this after refactoring few parts.

[nicolaasuni]

I had to remove the SvgBlendModeTest.php as it is failing.
Tests in the test folder executed by phpunit should be pure unit tests. I plan to add them when the HTML part is complete.
Other tests, like the functional ones, should be separated and be able to run automatically in the current workflow.