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 <noreply@anthropic.com>

src/app/document.rs

@@ -27,6 +27,10 @@ pub(super) enum DynComponent {
     Distance,
     /// Angle from the last point, in degrees.
     Angle,
+    /// A scalar the command reads from the command line (a count, a radius,
+    /// a delta). Typed-only — it has no geometric live value derived from
+    /// the cursor unless the command supplies one via `dyn_live_value`.
+    Scalar,
 }
 
 /// A single editable dynamic-input field. `buffer == None` means the box