Smart Guides: Distance Guides follow what lies under a still pointer

One rule for the hover's context (PR review follow-ups): the target is
recomputed where the pointer is, or cleared, whenever what lies under it
can change while Alt is held.

- The tool hovers only where the pointer is on the canvas itself
  (`on_canvas`): under a floating panel, a menu, a popup or a dialog
  its hover ends (Tool::hover_ended), as when the pointer leaves the
  canvas or a pan, zoom or view-rotation press starts.
- The canvas keeps the hover's context: its rect, the view's centre,
  zoom and rotation, and the active document's id and revision (which
  moves with every edit, undo and selection change). When it changes
  under a still pointer, a tool that asks (Tool::wants_rehover; the
  Selection tool after an Alt hover) is hovered again at the pointer,
  so wheel, gesture, key or command scrolling and zooming, tab
  switches, edits and undos re-measure.
- Coming back onto the canvas re-hovers the tool.

canvas::tests::distance_guides_follow_what_lies_under_a_still_pointer:
a floating window over the pointer (fails without the on_canvas filter),
a 150 pt scroll that leaves empty artboard under it (fails without the
re-hover), a square made under it. select::tests::distance_guides_need_
alt_a_selection_and_smart_guides checks wants_rehover.

crates/engine/src/tooling.rs

@@ -444,6 +444,12 @@ impl Session {
         self.with_tool_cx(view, |t, cx| t.hover_ended(cx));
     }
 
+    /// Whether the active tool's hover depends on what lies under a still pointer
+    /// ([`vectorcraft_tools::Tool::wants_rehover`]).
+    pub fn tool_wants_rehover(&self) -> bool {
+        self.tool.wants_rehover()
+    }
+
     pub fn tool_busy(&self) -> bool {
         self.tool.busy()
     }