feat(input): scalar entry via dynamic input

Text-input steps (OFFSET distance, POLYGON sides, radii, counts) were
swallowed by the dynamic-input overlay: typed digits landed in the field
buffer and Enter committed a point pick, so the value never reached
on_text_input (issue #61). Now each text step shows one scalar box whose
value routes to on_text_input on commit; non-numeric prompts (names,
keywords) keep going to the command line so mixed text like "Bolt12"
isn't split.

OFFSET reworked: object first, then through-mode by default — the offset
follows the cursor at the perpendicular distance to the object. Typing a
number locks the magnitude (cursor then only picks the side); a value
typed and clicked without Enter is applied before the point pick.

The dynamic-input overlay and the command line now show the active
step's prompt and hold it until the step completes.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/command/mod.rs

@@ -232,6 +232,10 @@ pub enum DynField {
     Distance,
     /// An angle, shown in degrees, measured from the last point.
     Angle,
+    /// A typed scalar with no geometric meaning at the cursor — a count
+    /// (number of sides / segments) or any value the command reads purely
+    /// from the keyboard. Shown as a single typed box.
+    Scalar,
 }
 
 // ── Trait ─────────────────────────────────────────────────────────────────
@@ -379,6 +383,17 @@ pub trait CadCommand: Send {
     fn dyn_field(&self) -> DynField {
         DynField::Point
     }
+
+    /// Live value for the dynamic-input scalar box, derived from the cursor
+    /// world position. Lets a command drive a typed prompt by mouse — e.g.
+    /// OFFSET returns the perpendicular distance from the cursor to the
+    /// object being offset, so moving the cursor fills in the distance.
+    /// Returns `None` when the value can only be typed (a count, or a
+    /// distance with no reference yet). The string the host commits is this
+    /// value formatted; the command's own `on_text_input` parses it back.
+    fn dyn_live_value(&self, _cursor: Vec3) -> Option<f64> {
+        None
+    }
 }
 
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