Application callback.

Start the application before you load or instantiate anything: it owns the module cache and the node page budget. The supervision tree is deliberately shallow at this stage. Milestones M0-M4 cover decoding, validation, execution and linear memory, none of which owns long-lived mutable state outside a caller's process. wasm_engine exists from the start only because node-wide page accounting must be in place before linear memory can grow (M4): atomics pages are off-heap and invisible to max_heap_size, so nothing else would bound them.