Provides a slot-based mechanism for collecting arbitrary HTML content from anywhere in the template tree and flushing it at a designated injection point — independent of render order.

Lifecycle:

1. {Portal name="nav"} calls add('nav', $html, $id) during body render
2. {Portal name="nav" output=true} calls inject('nav') → emits portal_nav
3. parseTemplateBlocks() resolves portal_nav by invoking the registered callback, which flushes the accumulated HTML wrapped in a