#1065: clean-room DNG / CinemaDNG decoder crate

Problem: camera RAW is at 0% (docs/codec-parity.md, #383). CinemaDNG,
the one cinema RAW format with a public specification, could not be
read at all.

Fix: new L0 crate filmcraft-dng, written from the public DNG 1.4-1.7,
TIFF 6.0 / TIFF-EP, ITU-T T.81 and CinemaDNG specifications only (no
other decoder's source read; editions in the crate README):
- TIFF container with SubIFDs, bounded (64 IFDs, 4 levels, cycles end
  the walk) and checked offsets; the main raw IFD is the largest
  NewSubFileType 0 CFA / LinearRaw image.
- Raw data: 1-16-bit uncompressed (MSB-first packing) and lossless JPEG
  (process 14, predictors 1-7, 1-4 components, point transform, line
  restarts), strips and tiles, tiles decoded in parallel; allocations
  capped by size and by the data present.
- Processing per the DNG model: ActiveArea, LinearizationTable, per-site
  black levels with deltas, white levels, as-shot white balance with
  neutral highlight clipping, Malvar-He-Cutler Bayer demosaic (bilinear
  for other patterns), DefaultCrop, ColorMatrix / CameraCalibration /
  ForwardMatrix with illuminant interpolation, Bradford to linear
  Rec. 709, BaselineExposure; output scene-linear RGBA floats.
- CinemaDNG FrameRate, TimeCodes and ReelName tags.
- A test-only writer (feature `writer`) synthesises DNGs. Tests: raw
  samples bit-exact for every depth / predictor / layout, processing
  against hand-computed values, hostile files (every truncation, 12k
  mutations, cyclic IFDs, huge sizes, bad offsets), and LibRaw's
  unprocessed_raw as an external oracle reading the same samples
  (skipped when not installed).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

xtask/src/main.rs

@@ -49,6 +49,7 @@ const LAYERS: &[(&str, u8)] = &[
     ("mjpeg", 0),
     ("dnx", 0),
     ("apv", 0),
+    ("dng", 0),
     ("av1", 0),
     ("aac", 0),
     ("opus", 0),