Pick a horizontal or vertical dimension from where its text is placed (#345)

Between two points, or on a line, that don't line up with an axis, the
Sketch Dimension tool only offered the distance between them: an auto
dimension ignored where its text went. Now the placement picks what it
measures: across from the two (between the perpendiculars through them)
the distance between them, as before; past that, above or below them the
horizontal distance, beside them the vertical one. With no text, an
explicit type, or two ends already on an axis, nothing changes.

The tool previews the dimension from the cursor, so the choice follows
the pointer while placing it.

Fixes #257

Co-authored-by: Vitali M. <[email protected]>
Co-authored-by: bherbruck <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

docs/book/src/guide/sketching.md

@@ -108,6 +108,10 @@ midpoint, symmetry, fix, curvature (G2) and polygon. **Sketch Dimension** (D) ad
 aligned, horizontal, vertical, radius, diameter and angle dimensions. Each is a parameter
 (`d1`, `d2`, …), so it can hold an expression.
 
+Between two points, or on a line, that don't line up with an axis, where you place the
+dimension picks what it measures: across from them, the distance between them; above or below
+them, the horizontal distance; beside them, the vertical distance.
+
 For **Coincident**, pick two points, a point and a curve (in either order), or two
 distinct straight lines. Two lines join their closest pair of endpoints; the first
 picked line stays in place when the existing constraints allow it. Pick endpoints