Slides: a second Shift-click extends from the same anchor; the range clears cells and returns to the slides; a bad from is an error (#1105)

Review on #1107: read `from` with opt_usize_param, clear `sel.cells` and set
`target = Slides` as `slide.go` did, keep the anchor of the first Shift-click.
A kittest drives the Shift and Ctrl+Shift clicks in the slide sorter.

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

crates/engine/src/lib.rs

@@ -155,6 +155,10 @@ pub struct Selection {
     pub text: Option<TextSel>,
     /// Selected table cells: (table, (r0, c0), (r1, c1)).
     pub cells: Option<(ShapeId, (usize, usize), (usize, usize))>,
+    /// Slide-list range last made with Shift-click: `(anchor, end)`. A further Shift-click extends from
+    /// the anchor while the current slide is still `end`; moving to another slide drops it.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub range_anchor: Option<(usize, usize)>,
     pub target: Target,
 }