php-in-php: VmGzStreamPure — drop libz gzFile FFI from gz* streams (#8936)

[PurHur]

Summary

• Add ext/standard/VmGzStreamPure.php — buffered plaintext read/write with VmZlib::gzencode/gzdecode on open/close
• Shrink VmGzStreamNative.php to a thin delegate (removed ~158 lines of libz gzFile FFI)
• php-src ref: ext/zlib/zlib.c (php_gzopen, stream ops)

Closes #8936

Runtime shrink

File	Before	After
VmGzStreamNative.php	~202 lines (libz FFI)	~44 lines (delegate)

FFI symbols removed: gzFile, gzopen, gzwrite, gzread, gzclose from VmGzStreamNative.

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit --filter VmGzStreamRuntimeShrinkTest
php bin/vm.php test/repro-maintainer/gz_stream_php_in_php_probe.php
'

PHPUnit 9.6.21 by Sebastian Bergmann and contributors.

...line1
line2
line1
line2
..                                                               5 / 5 (100%)

OK (5 tests, 26 assertions)
hello

make bootstrap-selfhost-vm-driver-execute-probe
php script/bootstrap-inventory.php --check
php script/check-selfhost-spine-coverage-sync.php

All green.

Note

Compression still routes through VmZlibNative libz FFI until #8837 lands pure-PHP deflate; this PR removes the separate gzFile stream FFI layer per issue scope.

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