Sketch: accept redundant but consistent constraints (#687) (#699)

A clover of same-size rectangles drawn around a shared corner was refused as over-constrained.

Constraints that remove no degree of freedom but still hold (zero residual) are now kept and shown dimmed as redundant. Conflicting ones are still refused. A redundant driving dimension is refused with "already determined by ...". Typed sizes become driven (reference) dimensions. Drawing no longer adds Horizontal/Vertical to a side that is already implied through two coincident corners. The solver reports redundant constraints using a Jacobian row-dependence test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/dim_view.rs

@@ -128,7 +128,7 @@ pub fn begin_dimension(app: &mut SolveApp, entities: Vec<String>, at: Vec2) -> R
     if draft.driven {
         PLACEMENT.with(|preview| *preview.borrow_mut() = None);
         app.run("sketch.dimension", json!({"entities": draft.entities, "text_at": [at.x, at.y], "driven": true}))?;
-        app.set_status("That dimension would over-constrain the sketch, so it was added as a driven (reference) dimension", false);
+        app.set_status("The sketch already determines that dimension, so it was added as a driven (reference) dimension", false);
         return Ok(());
     }
     EDIT.with(|edit| *edit.borrow_mut() = Some(Edit { param, expr, at: None, fresh: FRESH, wait: 10, draft: Some(draft), live: None }));