Pressing HDR no longer swaps the layout in one frame. The SDR curve squeezes
into the HDR layout's SDR section as the HDR section and legend open, and
Visualize HDR and HDR Headroom slide open with it (LR-EDIT-HDR-MODE); turning
HDR off runs it back. An SDR edit never shows an HDR histogram.
The flow lives in its own module, panels/edit/hdr_flow.rs, with small hooks in
histogram.rs and sliders.rs.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>