A linear DNG in and out: the writer, and a three-sample RawImage

dr-export gains write_linear_dng — LinearRaw, DNG 1.4, u16 samples at
the sensor's scale, the body's matrices with their illuminants, the
as-shot neutral, the EXIF block an export writes — streamed strip by
strip through a closure so the composite is never held (FR-MRG-11). The
tiff crate's directory is a map, so PhotometricInterpretation is written
over what new_image set, which is the trick the S15.1 spike thought it
had to hand-roll around. The test reads the file back through rawler.

dr-decode's RawImage carries samples_per_pixel (a linear DNG is 3), the
body's profile with its calibrations mapped back to EXIF illuminant
codes, and the cleaned make and model. The GPU uploads a three-sample
image as it is, normalised by black and white like a photosite, through
a full f16 conversion — subnormals kept, because a 14-bit LSB sits at
f16's smallest normal and rounding it to zero would crush exactly the
shadows the file was written to keep.

core/dr-export/src/lib.rs

@@ -24,6 +24,7 @@
 
 use dr_types::{ColourSpace, ExportFormat, ExportSettings};
 
+mod dng;
 mod encode;
 mod error;
 mod exif;
@@ -33,6 +34,7 @@ mod name;
 mod sharpen;
 mod size;
 
+pub use dng::{write_linear_dng, DngProfile};
 pub use error::ExportError;
 pub use metadata::SourceMetadata;
 pub use name::{resolve_name, NameContext};