Description of change

Update Container.parent type from Container to Container | null to match the actual runtime behavior where parent can be null when a container is not attached to a parent.
Why necessary: The current type forces developers to use non-null assertions (!) or disable ESLint warnings when checking if a container has a parent, even though the runtime value can legitimately be null.
Example:

// Current behavior requires non-null assertion or ESLint disable
let current = sprite;
while (current.parent!) { // ESLint: Unnecessary non-null assertion
    current = current.parent!;
}

// With the fix, natural null checking works
let current = sprite;
while (current.parent) { // Clean, no ESLint warnings
    current = current.parent;
}

This change aligns the TypeScript types with the actual runtime behavior where parent is initialized as null and can be null when containers are detached from their parent.

Pre-Merge Checklist

• Lint process passed (npm run lint)
• [-] Tests passed (npm run test) (the current dev branch seems to have many failing tests, I dont think its related)