EXR export: no compand / tone mapping, and the output space's own curve

OpenEXR renders (Render Queue sequences and Save Frame As ProEXR) received
the frame after Project Settings > HDR, so Compand and Tone Map (the default
of every OCIO project) compressed highlights and changed values below 1.0
before the writer decoded them, always with the sRGB curve.

- RenderOpts::scene_output: the top-level comp converts to the output space
  without the HDR step (CPU and GPU walks, via Renderer::output_pipe).
- Pipe::output_linear: the writers decode with the output space's curve
  (the Working Gamma's when unmanaged) instead of a fixed sRGB curve.
- Render Queue EXR jobs and ProEXR set the flag; PSD layers, 8-bit stills,
  movies and the viewer keep the HDR step. The flag is part of the viewer
  frame and disk cache keys (opts_hash).

Golden hashes, taken before the change, pin PNG/TIFF/Targa bytes for
8/16/32 bpc x Clip/Compand/ToneMap, Clip-mode EXR samples with an sRGB
output, and the layered PSD bytes.

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

docs/architecture.md

@@ -407,7 +407,11 @@ inputs in 32 bpc. With a working space (sRGB, Rec. 709, Rec. 2020, Display P3; m
 from the published primaries), footage is converted from its colour profile (stream metadata or
 Interpret Footage, else sRGB) and the top-level comp is converted to the sRGB display. Linearize
 Working Space runs everything in linear light; Blend Colors Using 1.0 Gamma linearises only for
-blending. Layer cache keys include these settings.
+blending. Layer cache keys include these settings. Project Settings ▸ HDR (compand / tone
+mapping) is the last top-level step, for standard-dynamic-range displays and outputs; OpenEXR
+renders (sequences and ProEXR) skip it (`RenderOpts::scene_output`) 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.
 
 **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