Linear dimensional constraints: point flow, dynamic dimension, driving parameter

DCLINEAR, DCHORIZONTAL, DCVERTICAL and DCALIGNED follow the reference flow:
`Specify first constraint point or [Object] <Object>:`, `Specify second
constraint point:`, `Specify dimension line location:`, then `Dimension
text = …` and `Enter value or name and value <d1=100>:`. Linear reads
horizontal or vertical from the dimension line location the way a linear
dimension does; Object takes a line's, an arc's or a polyline segment's
ends. A miss reports `No valid constraint point found.` / `No object
found.` and asks again; another object type reports `Invalid selection for
Linear. Select a line, polyline segment or arc.`.

Each constraint creates a drawing parameter (`d1`, `d2`, …, or the typed
`name=expression`), drives the constraint through it, and draws a dynamic
dimension in constraint gray whose text follows CONSTRAINTNAMEFORMAT
(`d1=100.0000`); the dimension is associated with the constrained objects
and its text is refreshed after every solve. A typed value other than the
measured one moves the second point while the first stays, and so does a
later parameter edit. Deleting the constraint removes its dimension and,
when nothing else reads it, its parameter.

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

src/command.rs

@@ -1610,6 +1610,25 @@ pub enum CmdResult {
         multiple: bool,
         label: &'static str,
     },
+    /// Resolves a constraint-point pick against the live document and hands
+    /// it back through `CadCommand::accept_constraint_point`; a miss reports
+    /// `No valid constraint point found.` and asks again.
+    CheckConstraintPoint(CoincidentPick),
+    /// Adds a dimensional constraint between two constraint points together
+    /// with the dynamic dimension that shows it and the parameter
+    /// (`name` = `expression`) that drives it.
+    AddDimensionalConstraint {
+        kind: crate::scene::parametric_constraints::ConstraintKind,
+        first: crate::scene::parametric_constraints::ParametricRef,
+        second: crate::scene::parametric_constraints::ParametricRef,
+        first_point: DVec3,
+        second_point: DVec3,
+        location: DVec3,
+        axis: DVec3,
+        name: String,
+        expression: String,
+        label: &'static str,
+    },
     /// Adds a point or object symmetry relation around a picked line. The
     /// first reference and axis remain fixed during initial placement.
     AddSymmetricConstraint {
@@ -2453,6 +2472,16 @@ pub trait CadCommand: Send {
         false
     }
 
+    /// A constraint point the host resolved for a `CheckConstraintPoint`
+    /// pick, with its world position.
+    fn accept_constraint_point(
+        &mut self,
+        _reference: crate::scene::parametric_constraints::ParametricRef,
+        _point: DVec3,
+    ) -> CmdResult {
+        CmdResult::NeedPoint
+    }
+
     /// Include filled hatch / DXF SOLID regions in the entity hit-test.
     ///
     /// Most entity-pick commands operate on curve geometry and intentionally