FR-MRG-3: the composite is a RAW at the source's native scale

Camera-linear u16 samples on the first source's black-subtracted scale
with its white level, never rescaled to fill 16 bits, with its body,
matrices, illuminants and as-shot neutral carried — so the panorama is
developed afterwards as one photograph from the sensor's own numbers.
The only thing a warp cannot preserve is the colour filter array, and
the clause says so.

docs/panorama.md

@@ -220,8 +220,24 @@ the way S9 calibrates R1.
 
 ## 8. The output file
 
-FR-MRG-3. Scene-linear, ≥ 16 bits, wide gamut, the first source's capture
-metadata, named from the first source with a `-pano` suffix, beside it.
+FR-MRG-3. A linear DNG at the source's native scale: `u16` samples on the
+first source's black-subtracted scale, `WhiteLevel` = its white minus its
+black (13 023 for the 6D set: 15 070 − 2 047), `BlackLevel` = 0. Not rescaled
+to 65 535 — the sensor had 14 bits and the file says so, and a value the
+sensor could not have produced is not invented by a multiply. The first
+source's `Make`, `Model`, `UniqueCameraModel`, `ColorMatrix1/2`,
+`CalibrationIlluminant1/2`, `AsShotNeutral` and EXIF are carried, so the
+composite develops through the same profile as its sources. Named from the
+first source with a `-pano` suffix, beside it.
+
+Three samples per pixel rather than a CFA: the warp resamples, and there is no
+sensor grid to mosaic back onto. Nothing else about being a RAW is lost —
+no white balance, no curve, no matrix, no clip has been applied — and the
+photographer develops the panorama afterwards as one photograph.
+
+The sources are portrait frames in the 6D set: `Orientation` is applied
+before alignment (learned features are not rotation-invariant) and the
+composite is written upright with `Orientation = 1`.
 
 Two containers were candidates and S15.1 decided, on 2026-09-19: