EXR export: write the output space's chromaticities attribute (#3038)

* EXR export: write the output space's chromaticities attribute (#3037)

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

* EXR chromaticities: tests for unmanaged and PQ ProEXR, every space once

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

* EXR export: note why the comp background is added before decoding (review point)

CodeRabbit asked to convert Comp.background to the output space before
compositing. Every output path (Image::to_rgba8_over) adds the background to
the output-encoded frame as it is, so the EXR now matches the other outputs;
converting would make the EXR colour differ from the PNG of the same comp.
Comment only.

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

* Format the merged code with rustfmt

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

---------

Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/architecture.md

@@ -414,7 +414,9 @@ mapping) is the last top-level step, for standard-dynamic-range displays and out
 renders (sequences and ProEXR) skip it (`RenderOpts::skip_hdr_step`) and decode the frame with the
 output space's own curve (`Pipe::output_linear`), so they hold linear light in the output space's
 primaries; unmanaged projects decode with the Working Gamma's curve (sRGB at 2.2, a pure 2.4 power
-curve at 2.4). Unmanaged projects and Rec. 2100 PQ / HLG outputs never get the HDR step.
+curve at 2.4). Every OpenEXR file (sequences and ProEXR) names those primaries and the white point in its
+`chromaticities` attribute (`ColorSpace::exr_chromaticities`; Rec. 709 / sRGB primaries for the default
+and for unmanaged projects, Rec. 2100 PQ and HLG as BT.2020, the ACES spaces with the ACES white). Unmanaged projects and Rec. 2100 PQ / HLG outputs never get the HDR step.
 
 **Frame blending**: footage and precomp layers whose source time falls between source frames
 (rate conform, stretch, remap) blend the two neighbouring frames, by cross-fade (Frame Mix) or by

docs/file-format-parity.md

@@ -41,7 +41,7 @@ trips and renders, checked by tests, not against After Effects), **partial** (na
 | PNG | R/W | yes | yes (sequence) | good |
 | JPEG | R/W | yes | yes (sequence) | good |
 | TIFF | R/W | yes | yes (sequence) | good |
-| OpenEXR (multi-layer, Cryptomatte) | R/W | yes | yes (scan-line sequence, half or 32-bit float, every compression but HTJ2K) | good; HTJ2K compression fails (#482); a multi-layer frame decodes once for its picture and EXtractoR / Cryptomatte from the first frame on, except Media Replacement items, which are learned at their first frame, and files whose picture the `image` crate still decodes (#474); footage alone keeps only its picture's channels, though a single-part file still decompresses every channel's pixels; decoded channels are held as f32 |
+| OpenEXR (multi-layer, Cryptomatte) | R/W | yes | yes (scan-line sequence, half or 32-bit float, every compression but HTJ2K) | good; written files carry a `chromaticities` attribute for the output space's primaries and white; HTJ2K compression fails (#482); a multi-layer frame decodes once for its picture and EXtractoR / Cryptomatte from the first frame on, except Media Replacement items, which are learned at their first frame, and files whose picture the `image` crate still decodes (#474); footage alone keeps only its picture's channels, though a single-part file still decompresses every channel's pixels; decoded channels are held as f32 |
 | Photoshop PSD / PSB | R (footage, composition) / W (layers, sequence) | yes (layers, groups, masks, styles, smart objects) | Save Frame As ▸ Photoshop Layers; no Photoshop sequence output | good, from Adobe's published spec |
 | Illustrator `.ai`, PDF, EPS | R | yes (vector, Continuously Rasterize, Create Shapes; Composition and Composition - Retain Layer Sizes, #1536) | — | partial: no JPX / JBIG2 images, no Illustrator procset EPS |
 | SVG | R | yes | — | good; SVG markup on the clipboard pastes as a shape layer (26.3, #738); Illustrator's own clipboard data (PDF/AI) doesn't |