Position-based superphase/subphase enablement breaks under assembly reordering

[rozyczko]

Source: EasyReflectometryApp/Backends/Py/logic/layers.py:29-38 and logic/project.py:91-95 — DEEP_ANALYSIS.md §5 item 18 (and §3.7)

_assembly_type infers superphase/subphase purely from list position (first/last), and _update_enablement_of_fixed_layers_for_model hardcodes the same convention by index. Reordering assemblies via move up/down can therefore re-enable a "fixed" superphase thickness without re-running enablement.

Fix: Re-run enablement after reordering, or derive superphase/subphase from an explicit property rather than list position.