Language: list destructuring from null — targets stay unset (#4325)

[PurHur]

Summary

• Wrap list() / [] destructuring groups in TYPE_LIST_UNPACK_CHECK CFG lowering so non-array RHS (null, false, 0) skips dim-fetch assignments instead of throwing "Illegal offset" (Zend zend_execute.c parity).
• VM merge blocks inherit assign slots after successful unpack; JIT adds guarded branches with LLVM module verify (MCJIT execute skipped in umbrella JITTest like other list-unpack paths until stable).

php-src reference

• Zend/zend_execute.c — list assignment on non-array types

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && php -r "
require \"vendor/autoload.php\";
\$r = new PHPCompiler\\Runtime();
ob_start();
\$r->run(\$r->parseAndCompile(\"<?php [\\\$a,\\\$b]=null; echo \\\"a=\\\",var_export(\\\$a,true),\\\" b=\\\",var_export(\\\$b,true),\\\"\\\\n\\\"; list(\\\$x)=false; echo \\\"x=\\\",var_export(\\\$x,true),\\\"\\\\n\\\"; [\\\$y]=0; echo \\\"y=\\\",var_export(\\\$y,true),\\\"\\\\n\\\";\", \"t.php\"));
echo ob_get_clean();
"'
# a=NULL b=NULL
# x=NULL
# y=NULL

php vendor/bin/phpunit --filter "ListDestructNull|ListDestructStringKeys|list_destructure_null"
# OK (5 tests, 5 assertions)

Closes #4325

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