Report a missed first 2Points pick at once for Horizontal/Vertical

The first empty-space click of the 2Points flow was only resolved against
the document together with the second one, so "No valid constraint point
found." appeared one pick late. The command now hands the first pick to
the host (`CmdResult::CheckHorizontalPoint`), which resolves it right away:
a miss reports and re-asks for the first point, a hit moves on to the
second point.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/command.rs

@@ -1592,6 +1592,13 @@ pub enum CmdResult {
         direction: acadrust::types::Vector3,
         label: &'static str,
     },
+    /// Resolves the first 2Points pick of a Horizontal or Vertical constraint
+    /// against the live document so a miss is reported at once, before the
+    /// second point is asked for.
+    CheckHorizontalPoint {
+        kind: crate::scene::parametric_constraints::ConstraintKind,
+        pick: CoincidentPick,
+    },
     /// Adds a point or object symmetry relation around a picked line. The
     /// first reference and axis remain fixed during initial placement.
     AddSymmetricConstraint {