i15-1: Run air collections with no sample loaded

For calibration scans with air, we need to run a scan with no sample loaded. The i15-1 plugin will need to construct just a scan plan with no robot load/unload or centring.

Start by locating the i15-1 plugin and existing scan-plan construction paths. Compare how calibration scans handle robot loading, unloading, and centring; done means an air collection produces only a scan plan and performs none of those sample-handling steps.

Written by the indexing model from the issue text.