Ask the platform what colour the screen actually is

FR-DSP-8's acquisition half. `dr_plat::display` surveys the session's
displays and reduces each one's profile to an output space the pipeline
can encode into, stating the mechanism per display server as
FR-PLAT-LIN-2 requires:

  - X11 reads the `_ICC_PROFILE` / `_ICC_PROFILE_<n>` root-window
    properties, enumerating and numbering the outputs through RandR,
    which also yields the rectangles a window move is measured against.
  - Wayland binds `wp_color_manager_v1` and asks each `wl_output` for
    its image description, accepting either an ICC profile on a file
    descriptor or primaries stated as chromaticities.
  - Where neither answers, sRGB is assumed and the reason travels with
    it as data rather than into a log, so the About page can say which
    path the session is on.

A display profile is a measurement of one panel and is none of the four
spaces the pipeline knows. Rather than grow an ICC engine, the profile
is reduced to D50-adapted colorants and matched against the four; a
match that is merely nearest is marked as such and shown as such.

Verified on this machine: mutter 50 advertises the colour-management
global and reports eDP-1 as sRGB, and the same session forced onto X11
enumerates the output through RandR and correctly finds no atom.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/dr-types/src/colour.rs

@@ -39,6 +39,22 @@ pub struct Chromaticities {
     pub white: [f64; 2],
 }
 
+impl Chromaticities {
+    /// This gamut's linear RGB to the ICC profile connection space, row-major.
+    ///
+    /// The same three columns [`ColourSpace::to_pcs_xyz`] produces, for a set
+    /// of primaries that is *not* one of the four the pipeline knows. That is
+    /// the only reason this is public: a display's profile describes primaries
+    /// nobody chose, and the only way to ask which of the four it is nearest
+    /// is to put both through the same reduction and compare the numbers
+    /// (see `dr_plat::display`). Comparing raw xy pairs instead would be
+    /// wrong, because a profile's colorants have already been adapted to D50
+    /// and a space's published chromaticities have not.
+    pub fn to_pcs_xyz(&self) -> [f32; 9] {
+        narrow(mul(adaptation(white_xyz(self), PCS_D50), rgb_to_xyz(self)))
+    }
+}
+
 /// How a space maps linear light onto the numbers stored in a file.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum Transfer {
@@ -183,8 +199,7 @@ impl ColourSpace {
     /// defined at D50 and nowhere else — a profile carrying unadapted D65
     /// colorants describes a space nobody asked for.
     pub fn to_pcs_xyz(self) -> [f32; 9] {
-        let c = self.chromaticities();
-        narrow(mul(adaptation(white_xyz(&c), PCS_D50), rgb_to_xyz(&c)))
+        self.chromaticities().to_pcs_xyz()
     }
 
     /// The white-point adaptation folded into [`Self::to_pcs_xyz`], row-major.