Tone: neutral display tone map is the identity up to white (#1945)

The SDR shoulder in ToneMap::display_hdr ran at neutral Contrast, Whites
and Blacks too, so an unedited JPEG or PNG exported neutral 255, 254 and
253 one level darker and a white background never came out as 255. The
shoulder now shapes edited curves only; neutral sliders are the identity
up to white and clip above it. Edited curves render as before (V1).

Tests: the identity test runs to 1.0, a new pipeline test renders the
issue's neutral greys back exactly, the sRGB ramp round trip is exact,
and a guard keeps the shoulder on edited curves.

docs/process-versions.md

@@ -22,6 +22,11 @@ stronger Light sliders, issue #196) without changing a single photo someone has
 - Changes to how a file is decoded or which camera colour it gets change the source, not how settings
   are interpreted; the process doesn't reach that stage today.
 
+**Neutral display tone map** (JPEG, PNG and other display-referred sources with Contrast, Whites and
+Blacks at 0) is the identity up to white: the SDR shoulder no longer pulls 1.0 down to 0.9875 (8-bit
+255 to 254, issue #1945). It landed in place as a bug fix, since the tone map promised identity at
+neutral settings; edited curves keep the shoulder and their V1 look.
+
 ## Adding a process
 
 1. Add a variant to `Process` (`V2`), append it to `Process::ALL` (oldest first), give it its number in