Follow the canvas from one display to the next

The rest of FR-DSP-8. The develop session now carries the space its
canvas is encoded into, and `render` composes for it instead of for
sRGB — which is the whole of the change to the pixel path, because the
output space was always a parameter of composition and always entered
the structure hash. A display change is a recomposition.

The space is set on the way into every render rather than pushed when
the window moves, so a photograph opened while the window already sits
on the second monitor is right on its first frame instead of flashing
the wrong colour until the next poll.

Which display that is comes from sampling the window's position and
scale factor twice a second — Slint reports neither a move nor a
display change — and re-surveying only when they differ. Settings shows
what came back under ABOUT: the display, the space, why, and the other
monitors, because the failure FR-DSP-8 names is one that is invisible
from the display you are reading the page on.

Fractional scaling: the canvas is now rendered at the physical pixel
size of the box it occupies rather than the logical one, so the
compositor presents it 1:1. At 1.25 it was previously handed 1600
samples to fill 2000 device pixels, and the softness that produces
reads like a bad demosaic rather than like a scaling bug.

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

ui/dr-ui/ui/settings.slint

@@ -117,6 +117,25 @@ export component SettingsPage inherits Rectangle {
     in property <string> layout-class;
     in property <string> app-version;
 
+    /// TRACES: FR-DSP-8
+    /// The display showing the canvas, and the colour it is being given.
+    ///
+    /// These are the same kind of thing as the two above — a fact about the
+    /// session, not a setting — with one difference that earns them their own
+    /// row rather than a line in a log. FR-DSP-8's fallback is *defined* to be
+    /// sRGB where the display server will not say otherwise, and a photographer
+    /// being shown sRGB because their compositor has no colour-management
+    /// protocol has no other way to find that out. `display-colour` says which
+    /// of the acquisition paths this session is on, in words.
+    in property <string> display-name;
+    in property <string> display-colour;
+    /// The *other* monitors, where there are any. Empty on a single-display
+    /// desktop, which is why the row below is conditional: FR-DSP-8 is about
+    /// the second display, and the failure it names is invisible from the
+    /// first, so this page has to be readable about a monitor the reader is
+    /// not currently looking at.
+    in property <string> display-others;
+
     callback original-budget-changed(string);
     callback original-unlimited-toggled(bool);
     callback thumbnail-budget-changed(string);
@@ -708,10 +727,46 @@ export component SettingsPage inherits Rectangle {
                             Value { text: root.layout-class; horizontal-stretch: 1; }
                         }
 
+                        // TRACES: FR-DSP-8
+                        // Which display, and what colour it is being sent.
+                        HorizontalLayout {
+                            spacing: Theme.gap;
+                            Label { text: "Display"; }
+                            Value {
+                                text: root.display-name;
+                                horizontal-stretch: 1;
+                                overflow: elide;
+                            }
+                        }
+
+                        HorizontalLayout {
+                            spacing: Theme.gap;
+                            Label { text: "Display colour"; }
+                            Value {
+                                text: root.display-colour;
+                                horizontal-stretch: 1;
+                                // Wraps rather than elides: this is the one
+                                // value on the page that is a sentence, and
+                                // eliding it would cut off the half that says
+                                // *why* — which is the half FR-DSP-8 asks for.
+                                wrap: word-wrap;
+                            }
+                        }
+
+                        if root.display-others != "": HorizontalLayout {
+                            spacing: Theme.gap;
+                            Label { text: "Other displays"; }
+                            Value {
+                                text: root.display-others;
+                                horizontal-stretch: 1;
+                                wrap: word-wrap;
+                            }
+                        }
+
                         Caption {
-                            text: "Graphics and frame rate describe this session, "
-                                + "not a setting — they are here so a bug report "
-                                + "can quote them.";
+                            text: "Graphics, frame rate and display colour "
+                                + "describe this session, not a setting — they "
+                                + "are here so a bug report can quote them.";
                             wrap: word-wrap;
                         }
                     }