Stdlib: socket_select() via poll(2) thin FFI (#6395)

[PurHur]

Summary

• Implement socket_select() in ext/sockets (VM) using thin libc poll(2) via SocketsLibcThinAbi — no new runtime/ C and no host socket_select delegation.
• By-ref $read/$write/$except write-back, Zend TypeError/ValueError messages, zero-timeout idle/ready on socket_create_pair.
• php-src ref: ext/sockets/sockets.c PHP_FUNCTION(socket_select).

Closes #6395

Test plan

• Issue repro (Zend + VM):

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/issue_6395_socket_select.php
php bin/vm.php test/repro/issue_6395_socket_select.php'

true
idle=0 remaining=0
ready=1 remaining=1
data='hi'
TypeError: … must only have elements of type Socket, string given

• vendor/bin/phpunit test/compliance/VMTest.php --filter stdlib/socket_select OK
• BuiltinByRefParamsTest::testSocketSelectArraysByRef OK
• JIT deferred (same pattern as socket_set_nonblock / socket_create_pair)

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