Fix bootstrap stdlib filesystem AOT under self-host stub gating (#1116)

[PurHur]

Summary

• Register mkdir, file_put_contents, unlink, and rmdir in SelfHostBuiltinPolicy::CATEGORY_FILESYSTEM so they keep real JIT lowering when PHP_COMPILER_SELFHOST_AOT=1 (previously stubbed to no-op ExternalMethod, producing 00 instead of 11).
• Add BootstrapPhaseCTest::testStdlibFilesystemAotLinkAndExecute for Phase C link+execute coverage.
• Sync SelfHostBuiltinPolicyTest auto-stub batch count (32) and assert filesystem builtins are not stubbed.

Closes #1116. Supersedes stale #1134 (fopen/fread/fclose landed via #1141).

Test plan

• PHP_COMPILER_SELFHOST_AOT=1 php bin/compile.php -o /tmp/fs test/bootstrap-aot/stdlib_filesystem.php && /tmp/fs → 11
• phpunit test/unit/SelfHostBuiltinPolicyTest.php
• phpunit test/aot/BootstrapPhaseCTest.php --filter testStdlibFilesystem
• ./script/bootstrap-aot-link.sh — stdlib_filesystem.php not in failure list (4 unrelated targets still failing on master)
• ./script/ci-fast.sh --filter SelfHostBuiltinPolicyTest

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