Merge the batch export, and settle the seven-branch merge

Resolves the last of the parallel work. Two conflicts worth recording,
because both were semantic rather than textual:

`render_for_export` gained a colour space on master while the batch branch
was rewriting the single-image export path around it. Kept both: the batch
request supersedes the synchronous path, and the space still has to be chosen
at render time because the conversion happens in the shader before the clip to
0..1. `render_open_frame` takes it as an argument rather than reaching for a
controller it does not hold.

The map-wait moved into `readback::await_mapping` on one branch while another
was editing the constant it used, so `READBACK_POLL_LIMIT` survived the merge
with no callers. Removed rather than left for clippy to find later.

1164 tests pass, clippy clean, fmt clean. Traceability 53.0% -> 54.3%.

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

ui/dr-ui/ui/app.slint

@@ -674,6 +674,19 @@ in property <bool> panel-visible: true;
     in property <string> export-label: "Export";
     in property <bool> export-busy: false;
     in property <string> export-status;
+    /// TRACES: FR-EXP-7
+    /// Whether an export would be queued for the server rather than written to
+    /// this device. The grid says the same thing as the develop button, but it
+    /// composes its own label around a selection count — so it is given the
+    /// fact and not the sentence.
+    in property <bool> export-to-server: false;
+    /// Whether a batch is running. The grid's export button becomes its cancel
+    /// button; there is nowhere else a long batch can be stopped from.
+    in property <bool> library-exporting: false;
+    /// Export the grid's selection in the background (FR-EXP-7).
+    callback library-export-selection();
+    /// TRACES: NFR-ARCH-3
+    callback library-cancel-export();
     /// Show or hide the develop column.
     callback toggle-panel();
     /// Export the image on screen, using the settings as they stand.
@@ -957,6 +970,12 @@ in property <bool> panel-visible: true;
                         root.paste-settings-to-selection();
                     }
 
+                    // TRACES: FR-EXP-7
+                    exporting: root.library-exporting;
+                    export-to-server: root.export-to-server;
+                    export-selection => { root.library-export-selection(); }
+                    cancel-export => { root.library-cancel-export(); }
+
                     sweep-done: root.library-sweep-done;
             sweep-total: root.library-sweep-total;