Feat: DIMALIGNED (DAL) and DIMDIAMETER (DDI) dimension commands

DIMALIGNED: 3-click flow (p1 → p2 → dim line position) → DimensionAligned.
Rubber-band preview shows extension lines and dim line offset.

DIMDIAMETER: 3-click flow (center → arc point → text position) → DimensionDiameter.
Previews the diameter chord line.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/annotate/diameter_dim.rs

@@ -0,0 +1,91 @@
+// DIMDIAMETER command — diameter dimension for circles and arcs.
+
+use acadrust::entities::{Dimension, DimensionDiameter};
+use acadrust::types::Vector3;
+use acadrust::EntityType;
+use glam::Vec3;
+
+use crate::command::{CadCommand, CmdResult};
+use crate::scene::wire_model::WireModel;
+
+enum Step {
+    CenterPoint,
+    ArcPoint(Vec3),
+    TextPoint { center: Vec3, arc_pt: Vec3 },
+}
+
+pub struct DiameterDimensionCommand {
+    step: Step,
+}
+
+impl DiameterDimensionCommand {
+    pub fn new() -> Self {
+        Self { step: Step::CenterPoint }
+    }
+}
+
+impl CadCommand for DiameterDimensionCommand {
+    fn name(&self) -> &'static str { "DIMDIAMETER" }
+
+    fn prompt(&self) -> String {
+        match self.step {
+            Step::CenterPoint    => "DIMDIAMETER  Specify center point:".into(),
+            Step::ArcPoint(_)    => "DIMDIAMETER  Specify point on circle:".into(),
+            Step::TextPoint {..} => "DIMDIAMETER  Specify dimension line location:".into(),
+        }
+    }
+
+    fn on_point(&mut self, pt: Vec3) -> CmdResult {
+        match self.step {
+            Step::CenterPoint => {
+                self.step = Step::ArcPoint(pt);
+                CmdResult::NeedPoint
+            }
+            Step::ArcPoint(center) => {
+                self.step = Step::TextPoint { center, arc_pt: pt };
+                CmdResult::NeedPoint
+            }
+            Step::TextPoint { center, arc_pt } => {
+                let mut dim = DimensionDiameter::new(v3(center), v3(arc_pt));
+                dim.base.definition_point  = v3(arc_pt);
+                dim.base.text_middle_point = v3(pt);
+                dim.base.insertion_point   = v3(pt);
+                dim.leader_length = arc_pt.distance(pt) as f64;
+                dim.base.actual_measurement = dim.measurement();
+                CmdResult::CommitAndExit(EntityType::Dimension(Dimension::Diameter(dim)))
+            }
+        }
+    }
+
+    fn on_enter(&mut self) -> CmdResult { CmdResult::Cancel }
+
+    fn on_mouse_move(&mut self, pt: Vec3) -> Option<WireModel> {
+        match self.step {
+            Step::CenterPoint => None,
+            Step::ArcPoint(center) => Some(preview_line(center, pt)),
+            Step::TextPoint { center, arc_pt } => {
+                let far = center + (center - arc_pt); // opposite point on circle
+                Some(preview_line(far, pt))
+            }
+        }
+    }
+}
+
+fn v3(p: Vec3) -> Vector3 {
+    Vector3::new(p.x as f64, p.y as f64, p.z as f64)
+}
+
+fn preview_line(a: Vec3, b: Vec3) -> WireModel {
+    WireModel {
+        name: "dimdia_preview".into(),
+        points: vec![[a.x, a.y, a.z], [b.x, b.y, b.z]],
+        color: WireModel::CYAN,
+        selected: false,
+        pattern_length: 0.0,
+        pattern: [0.0; 8],
+        line_weight_px: 1.0,
+        snap_pts: vec![],
+        tangent_geoms: vec![],
+        key_vertices: vec![],
+    }
+}