Bootstrap: literal GD2 defaults so Zend spine AOT can pass VmGd (#22642)

[PurHur]

Summary

• Exclusive gen-0 refresh r18 died at ext/gd/VmGd.php with #3803 (VmGdGd::GD2_CHUNKSIZE / GD2_FMT_RAW parameter defaults).
• Spine require order compiles VmGd.php before VmGdGd.php, so cross-class const defaults are not foldable under incremental AOT.
• Use matching literals (128 / 1) and guard with VmGdGd2LiteralDefaultsTest.

Continues in-flight pillar-1 work from #22717 / closed #22642 (Lane A: no eligible language IR issues this run; stdlib/ext queue is owned elsewhere).

Refs #22642 · #10533 (pillar 1 / north-star5-fast)

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter VmGdGd2LiteralDefaultsTest'
# OK (2 tests, 7 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php -r "
require \"vendor/autoload.php\"; require \"lib/Runtime.php\";
\$rt = new PHPCompiler\\Runtime(PHPCompiler\\Runtime::MODE_AOT);
\$rt->parseAndCompileFile(\"ext/gd/VmGd.php\");
echo \"PARSE_OK\\n\";
"'
# PARSE_OK

Test plan

• Unit guard VmGdGd2LiteralDefaultsTest
• AOT parseAndCompileFile(ext/gd/VmGd.php) OK
• Next exclusive Zend spine refresh should get past VmGd.php (expect later cross-unit #3803 or OOM — not this file)

Made with Cursor

[PurHur]

Merged a8acf4ccc.

Post-merge: vendor/bin/phpunit --filter VmGdGd2LiteralDefaultsTest → OK (2 tests).

Pillar 1 still red on master (north-star5-fast 4f-m fingerprint mismatch; gen-0 unverified-restamp). Next exclusive Zend spine refresh should get past VmGd.php; expect later #3803 cross-unit defaults or the known ~28 GiB OOM (#22642 analysis). Did not launch exclusive this run — 2× php-compiler:22.04-dev peers already up.

#10533 comments are locked (2500+); trust probe lives on this PR + #22717.