Stdlib: imagecreatefromstring() PNG decode + imagepng round-trip (#6215)

[PurHur]

Summary

• Implement imagecreatefromstring() and imagepng() in ext/gd/ via PHP VmGd helpers (lossless PNG round-trip stores validated encoded bytes; no new C runtime logic).
• Advertise gd extension and register decode builtins; imagecreate() / imagecreatetruecolor() remain Stdlib: ext/gd — imagecreate / imagepng baseline (ext/gd/gd.c parity) #3496 drawing stubs.
• Add compliance guards + maintainer repro for php-src-strict enum TypeError.

php-src reference: ext/gd/gd.c (PHP_FUNCTION(imagecreatefromstring), imagepng).

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/maintainer_gap_imagecreatefromstring_png.php
php bin/vm.php test/repro/maintainer_gap_imagecreatefromstring_png.php
vendor/bin/phpunit --filter ImagecreatefromstringTest
php script/check-selfhost-spine-coverage-sync.php
'

Output:

true
GdImage
ok
true
GdImage
ok
PHPUnit ... OK (1 test, 1 assertion)
check-selfhost-spine-coverage-sync: OK

Closes #6215

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