New command `photo.renderToDng` (Photo menu): each selected photo (at most
500) is rendered with its edits as a linear 16-bit float DNG
`<name>-Rendered.dng` next to its original (never overwriting; a demo photo
needs `dir`), added to the library stacked on top of the original, and the new
photos are selected. Colour space proPhoto by default (rec2020, adobeRgb,
displayP3, srgb). A batch holds a cancellable `renderDng` activity row with
progress; cancelling stops between photos and keeps the copies made so far;
a failing photo in a batch is listed and the rest carry on.
Stacked on #1047.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
