Replace global variable with static class property for PHP version cache

[Copilot]

Addresses code review feedback to eliminate global variable usage in PhpProject class.

Changes

• Replaced global $version with private static string|null $version = null
• Updated all references to use self::$version
• Changed isset() to explicit null check (self::$version === null) for proper null detection with initialized properties

class PhpProject
{
    private static string|null $version = null;

    private function php(...): Container {
        if (self::$version === null) {
            // Parse version from composer platform requirements
            self::$version = ...;
        }
        
        return dag()->container()->from(match (self::$version) {
            null => "{$image}:{$variant}",
            default => "{$image}:{self::$version}-{$variant}"
        });
    }
}

Improves testability by containing state within the class scope rather than global namespace.

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