Add stress example for include resolution + namespace fallback + function_exists

[nahime0]

Why

The include resolver, namespace fallback rules, and function_exists() behavior are individually covered, but the mixed program path is a good place for subtle mismatches.

Proposed stress program

<?php
namespace App;
use function App\Lib\helper;

require __DIR__ . "/lib.php";

echo function_exists("helper") ? "y\n" : "n\n";
echo function_exists("\\App\\Lib\\helper") ? "y\n" : "n\n";
echo helper() . "\n";

What to verify

• Include ordering matches PHP-visible availability.
• Unqualified function lookup respects namespace fallback rules.
• function_exists() agrees with the callable that name resolution eventually targets.
• No drift between include discovery and function-variant tracking.

Scope

Stress/regression coverage issue, not a confirmed compiler bug.