S15.1: rawler reads back a linear DNG, so that is the container

A hand-rolled 64×48 LinearRaw DNG — one IFD, 16-bit RGB, DNGVersion,
ColorMatrix1, AsShotNeutral — comes back through rawler 0.7 with cpp 3,
the samples in the order written and the matrix parsed into the camera
definition; CameraProfile::extract builds a profile from it. ImageMagick
reads the same bytes.

dr_decode::decode currently accepts the file as CFA and passes three
times the samples on, so the cpp == 3 branch is the decode work FR-MRG-3
needs, and the only decode work. panorama.md §8 records the result.

docs/panorama.md

@@ -167,7 +167,7 @@ the way S9 calibrates R1.
 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.
 
-Two containers are candidates and S15.1 decides:
+Two containers were candidates and S15.1 decided, on 2026-09-19:
 
 - **Linear DNG.** `PhotometricInterpretation = LinearRaw`, three samples per
   pixel, `ColorMatrix1` carried from the first source. Re-enters through
@@ -178,9 +178,21 @@ Two containers are candidates and S15.1 decides:
   the existing encoder with a different sample type, and nothing about it is
   uncertain.
 
-Either way the composite enters the pipeline as a non-CFA, *linear* source —
-`from_rgba8`'s sibling with `non_linear = false` and the base curve resolved
-from the carried matrix — and is developed as any RAW is.
+**Linear DNG.** [`examples/linear_dng.rs`](../core/dr-decode/examples/linear_dng.rs)
+hand-rolls a 64 × 48 `LinearRaw` DNG — one IFD, uncompressed 16-bit RGB,
+`DNGVersion`, `ColorMatrix1`, `AsShotNeutral`, `CalibrationIlluminant1` — and
+rawler 0.7 reads it back: `cpp 3`, the samples interleaved as written, the
+matrix parsed into the camera definition, and `CameraProfile::extract` builds
+the same profile it would for a camera file. ImageMagick's libraw reads the
+same bytes. What does *not* yet work is `dr_decode::decode`, which accepts the
+file as CFA and hands the pipeline three times the samples it expects: the
+`cpp == 3` branch is the work, and it is the only decode work.
+
+The composite therefore enters the pipeline as a non-CFA, *linear* source —
+`from_rgba8`'s sibling with `non_linear = false` and the colour matrix carried
+from the DNG — and is developed as any RAW is. The writer is the example's
+IFD, grown up: tiled rather than one strip (FR-MRG-11 encodes per chunk), and
+carrying the first source's EXIF in a sub-IFD as `dr-export` already does.
 
 ## 9. Interaction