Some final notes:

1. Gwenview's "Configure" panel, and this issue topic, say "(ICC) rendering intent". To be clear, its specifically the monitor rendering intent. An image can have an input, working and output profile/space, so it's better to be clear about this, even though in Gwenview's case there is not much room for confusion since it's just an image viewer.
2. "Scale all colors equally to fit them within the active color profile's color gamut" Yes, but: a. s/active color profile's/active monitor profile's/ b. This will only work if the monitor profile is a LUT-type profile with perceptual intent gamut mapping included. Matrix profiles, or LUT profiles without perceptual mappings, will not scale the colors - they will function the same way as "relative colorimetric" does, i.e. using those profiles, nothing will change when changing this intent setting. I suggest you mention this, it's not obvious.
3. "Squash colors" - that's true because it's vague. What this intent does, and I think it's more clear to phrase it this way, is that it shows out-of-gamut colors using their nearest in-gamut representations - in-gamut colors are not altered while out-of-gamut colors are clipped. i.e. (simplified) if you have an image of a red gradient which stretches from some imaginary value 0 (min) to 100 (max), and the monitor physically cannot show reds redder than 93, the image on screen when using the relative colorimetric intent will show accurate reds from 0-93, and all pixels redder than 93 will look like 93.

Made obsolete by a better solution: user-selectable rendering intent.

Looks good!

Could be because you omitted -d1.
Why not just use DisplayCAL?

For correct color management, the calibration curves must be loaded into the VCGT and the color-managed programs must find and use the ICC profile. If either is missing, or are incompatible, colors won't be correct.

@ngraham jumping ship is not in my nature - filing bug reports and enhancement requests to improve the situation is ;)

I'm told the X11 atom doesn't/won't work in Wayland, so polling colord would be good, and it's always good to have the fool-proof way of being able to point to the ICC file manually and explicitly.

Ideally, a program should let the user choose an ICC in one of two ways:

• Auto-detect the system profile
• Manually point to ICC file

Very good!

I think this is one of those cases where appearing to be more user-friendly is in fact having the opposite effect. Everyone familiar with what this does is familiar with the correct term, while "color rendering" is vague at best.

Great to see this!

It's no accident this exists:
https://xkcd.com/1882/
;)

That would be fantastic!

You mix here a profiling question (LUT/matrix+shaper profiles for monitors) with a workflow question (rendering intent).

1. It is unreasonable to force an intent which alters the appearance of the whole image in an image viewer. In-gamut colors should not suffer in an image viewer due to the monitor being unable to display a few out-of-gamut colors.
2. Matrix profiles don't support the perceptual intent. As for LUT profiles, DisplayCAL, the go-to profiling program for Linux, does not include perceptual mapping by default, the user must do that manually. Image viewers will therefore fall back to relative colorimetric. This is what happens in Gwenview and why more people haven't complained.
3. That leaves only those people who use LUT profiles and who do have perceptual mappings in their monitor profiles with the problem.
4. "ICC recommends the perceptual rendering intent for most situations" @behrmann please link to the source where the ICC recommends using the perceptual intent for viewing images displayed on screen. If such a recommendation exists, I would also like to read why they recommend it and how they justify distorting the appearance of the whole image as the optimal solution in an image viewer.
5. "it [perceptual] is the most often used default" is an odd claim. Most often used where? Printing - perhaps, difficult to say. Viewing? I don't think so. Not in GIMP, not in RawTherapee, not in Oyranos (I just installed it in a clean system and it defaulted to relative), not in Adobe Photoshop as far as I could find. Besides, Adobe's own docs have this to say about the default intents for printing (not viewing), "(Perceptual] is the standard rendering intent for the Japanese printing industry", and, "[Relative colorimetric] is the standard rendering intent for printing in North America and Europe."
6. Ideally, Gwenview should present the user with a choice of rendering intent and black point compensation, without needing a color management service installed, but that is outside the scope of this patch.

It took me an hour to fish out the old gear and retrieve the ICC, an equivalent of which I already posted almost two years ago:
https://bugs.kde.org/show_bug.cgi?id=359909#c0