Stdlib: imagecreatetruecolor raster + PNG emit for ext/gd (#3496)

[PurHur]

Summary

• Implement imagecreatetruecolor(), imagecolorallocate(), imagefill(), and imagedestroy() in ext/gd/ using a PHP-in-PHP truecolor raster model (no libgd drawing).
• Add VmGdPng minimal PNG encoder so imagepng() emits valid bytes from raster canvases (decode-from-string path unchanged).
• Spine + inventory sync for four new ext/gd/ units.

php-src reference

• php/php-src ext/gd/gd.c — PHP_FUNCTION(imagecreatetruecolor), imagecolorallocate, imagefill, imagepng

Verification

php bin/vm.php test/repro/maintainer_gap_gd_imagecreatetruecolor.php
# ok

php bin/vm.php test/compliance/cases/stdlib/gd_imagecreate.phpt
# ok

php script/check-selfhost-spine-coverage-sync.php
# OK (spine covers all 4755 inventory files)

./script/phpunit.sh --filter 'BootstrapSelfhostBundleTest::testCompilerLibSpineSmokeBundleUnitCountAndKeyUnits'
# OK (1 test, 478 assertions)

Closes #3496

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