Split merge_ui::wire into one function per section

The 312-line wire() registered the panorama page's callbacks in three
comment-delimited sections. Lift each into its own fn: wire_start (the
"Merge to panorama" press, its fetch, and the DARKROOM_START_MERGE dev
entry point — all three only ever used together), wire_decision (confirm,
projection and border chips, the fill knobs), and wire_stop_and_leave
(abandon, close). wire() itself now just calls the three in order; S, C
and F stay generic on wire_start since sources/context/on_done are used
nowhere else.

ui/dr-ui/src/merge_ui.rs

@@ -92,11 +92,28 @@ pub fn wire<S, C, F>(
     S: Fn() -> Vec<crate::export::Source> + 'static,
     C: Fn() -> Option<Context> + 'static,
     F: Fn(&AppWindow) + 'static,
+{
+    wire_start(window, &ctl, gpu, sources, context, on_done);
+    wire_decision(window, &ctl);
+    wire_stop_and_leave(window, &ctl);
+}
+
+/// Start.
+fn wire_start<S, C, F>(
+    window: &AppWindow,
+    ctl: &Rc<MergeController>,
+    gpu: Option<dr_gpu::GpuContext>,
+    sources: S,
+    context: C,
+    on_done: F,
+) where
+    S: Fn() -> Vec<crate::export::Source> + 'static,
+    C: Fn() -> Option<Context> + 'static,
+    F: Fn(&AppWindow) + 'static,
 {
     let on_done = Rc::new(on_done);
     let gpu_for_start = gpu.clone();
 
-    // --- start -----------------------------------------------------------
     {
         let weak = window.as_weak();
         let ctl = ctl.clone();
@@ -243,7 +260,7 @@ pub fn wire<S, C, F>(
             };
             start(
                 window,
-                &ctl,
+                ctl,
                 gpu,
                 names,
                 MergeDestination::Local(dir),
@@ -265,8 +282,10 @@ pub fn wire<S, C, F>(
             }
         }
     }
+}
 
-    // --- the decision ----------------------------------------------------
+/// The decision.
+fn wire_decision(window: &AppWindow, ctl: &Rc<MergeController>) {
     {
         let weak = window.as_weak();
         let ctl = ctl.clone();
@@ -360,8 +379,10 @@ pub fn wire<S, C, F>(
             w.set_merge_stage("Filling the preview".into());
         });
     }
+}
 
-    // --- stopping and leaving --------------------------------------------
+/// Stopping and leaving.
+fn wire_stop_and_leave(window: &AppWindow, ctl: &Rc<MergeController>) {
     {
         let weak = window.as_weak();
         let ctl = ctl.clone();