Associate brightness/contrast with alignments (per-alignment B/C) #89

Problem

Brightness/contrast is not tied to individual alignments. This is tied to the autoseg workflow, assigning to Vijay.

Proposed

Let each alignment carry its own brightness/contrast.

Notes

KH's request (verbatim): "Brightness / contrast - are they associated with alignments? They should be, allow unique brightness/contrast for each alignment."
Today they are separate: B/C lives per section in section.bc_profiles keyed by the series-level series.bc_profile, while transforms live per section in section.tforms keyed by series.alignment. changeAlignment() (gui/main/field_widget_4_data.py) and changeBCProfile() (gui/main/field_widget_7_view.py) are independent, so changing the alignment does not change the B/C profile. The two are already parallel per-section keyed dicts, so associating them (e.g. switching the B/C profile alongside the alignment) is a natural fit. A named B/C-profile feature partly covers "unique B/C" but is not linked to alignments.