php-in-php: include_path stack compliance + BuiltinExecute on set/get (#3223)

[PurHur]

Summary

• Add include_path_stack VM/JIT compliance PHPTs covering set → include → restore (Stdlib: get_include_path() / set_include_path() / restore_include_path() — include path stack (bootstrap) #3223 bootstrap gate).
• Fix file_include_path*.phpt to avoid ?? (parser does not lower Expr_BinaryOp_Coalesce in PHPT bodies yet).
• Route get_include_path() / set_include_path() through BuiltinExecute::writeReturn so argc validation and stack mutation always run (Stdlib: VM builtins skip arg validation when return is discarded (returnVar null early-return) #5896 pattern).

C runtime: 0 lines added/removed (phpc_progress.c unchanged — SIGSEGV ABI stub only).

php-src reference

• ext/standard/basic_functions.c — get_include_path, set_include_path, restore_include_path
• main/php_open_temporary_file.c — PG(include_path) stack

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && vendor/bin/phpunit --filter include_path'

OK (12 tests, 12 assertions)

Issue repro (VM):

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php -r "
foreach ([\"get_include_path\",\"set_include_path\",\"restore_include_path\"] as \$fn) {
  echo \$fn, \"=\", function_exists(\$fn) ? \"yes\" : \"no\", \"\\n\";
}
\$dir = sys_get_temp_dir() . \"/phpc_inc_\" . getmypid();
mkdir(\$dir);
file_put_contents(\$dir . \"/only_here.php\", \"<?php return \\\"found\\\";\");
\$old = set_include_path(\$dir . PATH_SEPARATOR . get_include_path());
\$r = include \"only_here.php\";
restore_include_path();
@unlink(\$dir . \"/only_here.php\");
@rmdir(\$dir);
echo \"include=\", var_export(\$r, true), \"\\n\";
"'

get_include_path=yes
set_include_path=yes
restore_include_path=yes
include='found'

Closes #3223

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