feat(dynamic-input): per-step DynSpec framework + guide overhaul

Introduce a per-command, per-step dynamic-input description so each step
shows the value boxes and guide geometry that make sense for it, instead
of a one-size-fits-all distance/angle pair.

- command: add DynSpec{anchor, fields(role+label), guide}, DynRole
  (X/Y/Z/Distance/Angle/Radius/Diameter/Width/Height/Factor/Count with
  label + value scaling), DynGuide(None/Polar/AxisDelta/Radius/RectSides)
  and DynAnchor. New dyn_spec() trait method defaults to None so commands
  on the legacy dyn_field() path are unchanged.
- overlay: redraw the dynamic-input layer by guide — polar reference line
  + angle arc, single radius line, dotted axis legs, or a cursor row —
  and place each box by its role. Angle reads as the unsigned magnitude
  of the short arc; a typed polar angle takes the sign of the cursor side.
- app: route fields/guide/anchor through the tab; resolve points from the
  step anchor and de-scale typed values by role (typed diameter -> radius).
- circle: CR shows a radius value on one line to the cursor; CD shows the
  diameter (twice the cursor radius). No angle arc on radius steps.
- pline/mline: keep polar dynamic input on vertex steps that also accept
  keyword letters via point_step_accepts_keywords().

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/modules/home/draw/circle.rs

@@ -189,6 +189,21 @@ impl CadCommand for CircleCommand {
             StepCR::Radius(_) => DynField::Distance,
         }
     }
+
+    fn dyn_spec(&self) -> Option<crate::command::DynSpec> {
+        use crate::command::{DynAnchor, DynFieldSpec, DynGuide, DynRole, DynSpec};
+        match self.step {
+            // Center is a normal first-point pick.
+            StepCR::Center => None,
+            // Radius: a single R value with one dotted line from the centre to
+            // the cursor (no angle, no axis legs).
+            StepCR::Radius(c) => Some(DynSpec {
+                anchor: DynAnchor::Point(c),
+                fields: vec![DynFieldSpec::new(DynRole::Radius)],
+                guide: DynGuide::Radius,
+            }),
+        }
+    }
 }
 
 // ── Command: Center, Diameter ──────────────────────────────────────────────
@@ -262,6 +277,20 @@ impl CadCommand for CircleCDCommand {
         }
         None
     }
+
+    fn dyn_spec(&self) -> Option<crate::command::DynSpec> {
+        use crate::command::{DynAnchor, DynFieldSpec, DynGuide, DynRole, DynSpec};
+        match self.step {
+            StepCR::Center => None,
+            // Diameter: the box shows/accepts twice the cursor radius; resolved
+            // back to a radius point by the host (role scaling).
+            StepCR::Radius(c) => Some(DynSpec {
+                anchor: DynAnchor::Point(c),
+                fields: vec![DynFieldSpec::new(DynRole::Diameter)],
+                guide: DynGuide::Radius,
+            }),
+        }
+    }
 }
 
 // ── Command: 2-Point ──────────────────────────────────────────────────────

src/modules/home/draw/mline.rs

@@ -63,6 +63,13 @@ impl CadCommand for MlineCommand {
         self.waiting_scale || !self.points.is_empty()
     }
 
+    fn point_step_accepts_keywords(&self) -> bool {
+        // The vertex steps accept J / S keywords but are point picks, so keep
+        // polar dynamic input. The scale prompt (`waiting_scale`) is genuine
+        // text entry and is excluded.
+        !self.waiting_scale && !self.points.is_empty()
+    }
+
     fn on_text_input(&mut self, text: &str) -> Option<CmdResult> {
         // Waiting for scale value
         if self.waiting_scale {

src/modules/home/draw/polyline.rs

@@ -253,6 +253,12 @@ impl CadCommand for PlineCommand {
         !self.vertices.is_empty()
     }
 
+    fn point_step_accepts_keywords(&self) -> bool {
+        // Each segment is a point pick that also accepts A / L / C / U, so the
+        // polar dynamic-input distance/angle stays visible.
+        !self.vertices.is_empty()
+    }
+
     fn on_text_input(&mut self, text: &str) -> Option<CmdResult> {
         match text.trim().to_uppercase().as_str() {
             "A" | "ARC" => {