Let a manual collection be put in the order it is meant to be seen in

`collection_members.position` and `Sort::CollectionPosition` have been in the
catalog since collections were, and nothing above dr-catalog has ever written
or read either: `collections::set_order` had no callers, and the grid ordered
everything by capture time whatever it was scoped to — dr-ui does not construct
a `Query` at all, it has its own `GRID_ORDER` constant. So a manual collection
was a set with an order nobody could see or change.

Three pieces, because it could not be fewer:

`grid_order_for` decides the ordering from the scope, and both readers take it
from there. That is the load-bearing part. An ordinal only names a photograph
relative to an ordering, so the window read and the span read have to agree —
a shift-click resolved through a different ORDER BY than the cells were drawn
with selects a different run than the one on screen, and the user finds out
when the export runs. `read_ids_span` already stated that invariant about
`GRID_ORDER`; this widens it to an ordering that depends on the scope.

Only a single manual collection has one. A set draws its descendants' images
too, and two children's positions are unrelated integers that interleave
arbitrarily; a smart collection has no member rows to carry a position at all.
Both fall back to capture time and refuse the drop rather than pretending.

The drop is on the cell, on whichever half of it the finger landed — the
trailing edge is the only way to name the last place in a collection, since
there is no cell beyond the last one to drop in front of.

`reordered` is pure and the membership is rewritten whole. `set_order` sets the
positions it is given and leaves the rest, so a partial write would interleave
the moved run with rows nobody touched; and it is read unfiltered, so what the
filter is hiding keeps its place relative to what the user can see.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/ui/app.slint

@@ -310,6 +310,14 @@ export component AppWindow inherits Window {
     /// TRACES: FR-CAT-5
     callback library-clear-selection();
     callback library-select-all();
+    /// TRACES: FR-CAT-7
+    /// Whether the grid is showing something with a manual order to change —
+    /// a single manual collection, not a set and not a saved filter.
+    in property <bool> library-reorderable: false;
+    /// Move the selection beside the photograph at this row of the loaded
+    /// window: after it where the drop landed on the cell's trailing half,
+    /// before it otherwise.
+    callback library-reorder-to(int, bool);
     callback library-collection-from-selection(string);
     /// TRACES: FR-CAT-6
     in property <string> library-range-from;
@@ -1442,6 +1450,8 @@ in property <bool> panel-visible: true;
                     toggle-date-range() => { root.library-toggle-date-range(); }
                     clear-selection() => { root.library-clear-selection(); }
                     select-all() => { root.library-select-all(); }
+                    reorderable: root.library-reorderable;
+                    reorder-to(row, after) => { root.library-reorder-to(row, after); }
                     collection-from-selection(name) => {
                         root.library-collection-from-selection(name);
                     }