Owns callback-backed MLIR external interface models.

Every attachment has a dedicated BEAM process. Native MLIR work invokes the callbacks from a context worker and waits through Kinda's callback runtime; the BEAM scheduler running the callback never executes native MLIR work that waits for itself. The external model is owned by the MLIR context. Destroying the context releases the native callback state and stops the attachment process.

Normally attachments are declared with Beaver.Slang.defop/2. The interface modules also expose attach/4 for dynamic operation definitions that are not written with Slang.