VarBuilder.flattenTag

Flattens a tag frame into a var according to SDL tag rules.

The flattening depends on the combination of values, attributes, and children:

• No values, no attrs, no children → empty object
• Single value, no attrs, no children → just the value
• Multiple values, no attrs, no children → array
• Single value + children → value becomes nested key
• Attributes only → object with @-prefixed keys
• Value + attributes → #value + @-prefixed keys
• Multiple values + attrs/children → #values array + other keys

Namespace, if present, is stored under "#ns".