A QNN backend runtime: the dlopen'd backend library plus its backend and device handles.

The HTP (NPU) is a shared SoC resource — use htp/0 to get a process-wide memoized runtime instead of creating one per caller. Explicit create/1 remains useful for the CPU backend or custom library paths (e.g. host-side testing).

close/1 waits for in-flight executes and returns {:error, :contexts_alive} while contexts created from this runtime are still open — close contexts first (or let the GC destructors handle teardown in dependency order).

Error shapes across the library: {:error, tag} for simple conditions (:runtime_closed, :graph_not_found, ...), {:error, {tag, detail}} for tagged details ({:dlopen_failed, msg}, {:unsupported_tensor_type, name}), and {:error, {:qnn, code, message}} for QNN backend errors.