Folds the parts of #995 that #999 lacked into #999:
- engine/media.rs: with a display profile, a diagnostic view keeps the
photo's histogram (taken before the overlay is drawn) and stays out of the
view cache; the clipping previews test sRGB values and are converted for
the display afterwards. Before, holding Alt on a P3 display swapped the
histogram for the clipping view's (mean luma 0.59 -> 0.999).
- Noise Reduction Luminance / Detail / Contrast show greyscale while
Alt-dragged (Overlay::for_control).
- Tests: diagnostic_views_keep_the_photos_histogram (P3 display; fails
without the media.rs change), an end-to-end headless drag that presses
and releases Alt mid-drag and checks the loupe pixels
(tests_alt_preview.rs), and every Alt-mapped id is a real control.
- Docs: NR row, What's New and ui-parity note; parity summary refreshed.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
