Stdlib: pfsockopen() persistent TCP sockets (#3384)

[PurHur]

Summary

• Implement pfsockopen() in ext/standard/pfsockopen.php by delegating connect + persistence to host \pfsockopen() and adopting the stream into VmFs (php-src ext/standard/fsock.c parity).
• Add VmFs host-resource refcounting so repeated persistent opens sharing the same underlying host stream fclose() safely (required when host returns identical resource for same host:port).
• Add compliance PHPT + PfsockopenBuiltinTest (including ephemeral-server reuse probe).

php-src reference

• ext/standard/fsock.c — PHP_FUNCTION(pfsockopen)
• main/streams/streams.c — persistent stream list (php_stream_popen)

PHP implementation

• ext/standard/pfsockopen.php — VM builtin
• ext/standard/VmPersistentSocket.php — stream URI helper
• ext/standard/VmFs.php — shared host-resource refcount on adopt/fclose

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php -r "var_dump(function_exists(\"pfsockopen\"));"
php bin/vm.php -r "var_dump(function_exists(\"pfsockopen\"));"
'
# bool(true)
# bool(true)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter PfsockopenBuiltinTest'
# OK (2 tests, 4 assertions)

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter stream_socket_client_context'
# OK (2 tests, 2 assertions) — refcount regression check

Closes #3384

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