Name the eye pass on the People screen

Once every image has been through the detector and only readings are
left — the state an already-indexed library is in the day the eye models
arrive — the button reads "Read eye state" rather than promising to
index, and the coverage line says what the faces are waiting for.

ui/dr-ui/ui/app.slint

@@ -406,6 +406,7 @@ export component AppWindow inherits Window {
     in property <string> identity-back-label: "‹ Library";
     in property <string> identity-coverage;
     in property <bool> identity-coverage-complete: false;
+    in property <bool> identity-coverage-read-only: false;
     in property <int> identity-picked: 0;
     /// The namesake merge the Identity screen is offering. See IdentityScreen.
     in property <string> identity-merge-offer-name;
@@ -1419,6 +1420,7 @@ in property <bool> panel-visible: true;
             back-label: root.identity-back-label;
             coverage: root.identity-coverage;
             coverage-complete: root.identity-coverage-complete;
+            coverage-read-only: root.identity-coverage-read-only;
             merge-offer-name: root.identity-merge-offer-name;
             merge-offer-faces: root.identity-merge-offer-faces;
             name-revision: root.identity-name-revision;

ui/dr-ui/ui/identity.slint

@@ -270,6 +270,12 @@ export component IdentityScreen inherits Rectangle {
     // hides the reason the rest are outstanding.
     in property <string> coverage;
     in property <bool> coverage-complete: false;
+    /// TRACES: FR-CULL-8a
+    /// Every image has been through the detector and only faces are left to
+    /// read — the eye pass over a library indexed before the eye models
+    /// existed. The same sweep, fetching the same originals; the button says
+    /// what it will actually do.
+    in property <bool> coverage-read-only: false;
     // No model on disk: face indexing cannot run at all (docs/faces.md §2.2).
     in property <bool> model-missing: false;
     in property <int> picked-count: 0;
@@ -616,7 +622,7 @@ export component IdentityScreen inherits Rectangle {
                     clicked => { root.confirm-all(); }
                 }
                 if !root.indexing && !root.model-missing && !root.coverage-complete: Button {
-                    text: "Index faces";
+                    text: root.coverage-read-only ? "Read eye state" : "Index faces";
                     clicked => { root.index-faces(); }
                 }
                 if root.indexing: Button {