feat(sketch): live drag that keeps its solution, ended by sketch.drag_end (#341)

* feat(sketch): constructive solving ahead of the numeric solver

Part 1 of splitting #251: the sketch crate only. A graph-constructive front
end (Owen 1991; Fudos & Hoffmann 1997) places what it can by ruler-and-compass
loci and cluster decomposition, Levenberg-Marquardt finishes and verifies the
rest, and the rank analysis stays authoritative for degrees of freedom. Solves
can follow a reference (the previous solution), so edits and drags keep their
branch instead of flipping.

New API: solve_with (hold, reference, analysis, budget) and
solve_after_value_edit; solve and solve_holding keep their signatures as
wrappers, so no caller changes here.

When the seed moves an arc's centre, the arc's unconstructed ends start carried
along (the arc translates) instead of left behind for LM to turn; found by the
reference test 20-slot-constrained.

Refs #203

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TWc1smHYwTSVrVc8SNvS44

* feat(doc): keep a sketch's solved state across value edits

Part 2 of splitting #251 (builds on part 1, the sketch crate):
- Sketch edits solve for the document's values continuing from the stored
  solved state (Document::solve_sketch), so a dimension edit keeps the
  sketch's branch instead of re-solving from scratch.
- A value edit (parameters, a configuration, the units) that would make a
  sketch that solved before fail or turn over is refused with the
  conflicting constraints named; otherwise the solved sketches are written
  back into the document (Model::adopt_solved_sketches), which changes what
  saved designs store.
- Later features are keyed by a sketch's solved geometry (to a 1e-6 mm
  quantum), so setting a value back reuses their earlier results.
- Solved sketches in a model state are shared (Arc) between states.

Refs #203

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TWc1smHYwTSVrVc8SNvS44

* feat(sketch): live drag that keeps its solution, ended by sketch.drag_end

Part 3 of splitting #251 (builds on part 2):
- sketch.drag follows a long move in short held steps, so the geometry keeps
  its branch, and reports how far it got (reached, at).
- live: true runs one frame of an interactive drag: the sketch is solved
  once and shown, later features keep their last result (marked stale)
  until sketch.drag_end or any command that reads the model evaluates it
  (CommandSpec::live, reads_model, doc_only).
- sketch.drag_end adds the constraint the drag let go on and evaluates the
  model once.
- Undo and redo evaluate a new document once (execute does) instead of
  twice.
- The viewport drives drags through live frames and drag_end; dimension
  text drags start only off sketch geometry.

Refs #203

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TWc1smHYwTSVrVc8SNvS44

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/cmd/edit.rs

@@ -8,8 +8,8 @@ use crate::params::{bad, bool_, num, str_, string_list};
 use crate::{EngineError, Result, Sel, Session, Snapshot};
 
 pub static COMMANDS: &[CommandSpec] = &[
-    CommandSpec::new("edit.undo", "Undo", undo).icon("undo").key("Ctrl+Z").noundo(),
-    CommandSpec::new("edit.redo", "Redo", redo).icon("redo").key("Ctrl+Y").noundo(),
+    CommandSpec::new("edit.undo", "Undo", undo).icon("undo").key("Ctrl+Z").noundo().doc_only(),
+    CommandSpec::new("edit.redo", "Redo", redo).icon("redo").key("Ctrl+Y").noundo().doc_only(),
     CommandSpec::new("edit.checkpoint", "Checkpoint", checkpoint)
         .noundo()
         .params("→ {checkpoint: id}: remember the design, active sketch and undo history (restore with edit.restore_checkpoint)"),
@@ -51,28 +51,35 @@ pub static COMMANDS: &[CommandSpec] = &[
 
 fn undo(s: &mut Session, _p: &Value) -> Result<Value> {
     let Some(prev) = s.undo.pop() else { return Err(EngineError::Other("nothing to undo".into())) };
-    let label = prev.label.clone();
-    s.redo.push(Snapshot { label: label.clone(), doc: s.doc.clone(), active_sketch: s.active_sketch, pending_moves: s.pending_moves.clone() });
-    s.doc = prev.doc;
-    s.active_sketch = prev.active_sketch;
-    s.pending_moves = prev.pending_moves;
-    s.selection.clear();
-    s.refresh();
+    let now = go_to(s, prev);
+    let label = now.label.clone();
+    s.redo.push(now);
     Ok(json!({"undone": label}))
 }
 
 fn redo(s: &mut Session, _p: &Value) -> Result<Value> {
     let Some(next) = s.redo.pop() else { return Err(EngineError::Other("nothing to redo".into())) };
-    let label = next.label.clone();
-    s.undo.push(Snapshot { label: label.clone(), doc: s.doc.clone(), active_sketch: s.active_sketch, pending_moves: s.pending_moves.clone() });
-    s.doc = next.doc;
-    s.active_sketch = next.active_sketch;
-    s.pending_moves = next.pending_moves;
-    s.selection.clear();
-    s.refresh();
+    let now = go_to(s, next);
+    let label = now.label.clone();
+    s.undo.push(now);
     Ok(json!({"redone": label}))
 }
 
+/// Put the session at `snap`; returns where it was, under the same label.
+fn go_to(s: &mut Session, snap: Snapshot) -> Snapshot {
+    let was = Snapshot { label: snap.label, doc: s.doc.clone(), active_sketch: s.active_sketch, pending_moves: s.pending_moves.clone() };
+    let same = std::sync::Arc::ptr_eq(&s.doc, &snap.doc);
+    s.doc = snap.doc;
+    s.active_sketch = snap.active_sketch;
+    s.pending_moves = snap.pending_moves;
+    s.selection.clear();
+    // A new document is evaluated by `execute`.
+    if same {
+        s.refresh();
+    }
+    was
+}
+
 fn checkpoint(s: &mut Session, _p: &Value) -> Result<Value> {
     Ok(json!({"checkpoint": s.checkpoint()}))
 }
@@ -477,8 +484,7 @@ fn redefine(s: &mut Session, p: &Value) -> Result<Value> {
     }
     trial.doc_mut().marker = Some(idx + 1);
     trial.refresh();
-    (spec.enabled)(&trial).map_err(|why| EngineError::Disabled(spec.id.to_string(), why))?;
-    let r = (spec.run)(&mut trial, &params)?;
+    let r = spec.run_nested(&mut trial, &params)?;
     let made = r.get("feature").and_then(Value::as_u64).ok_or_else(|| bad(cmd, format!("`{cid}` did not make a feature")))?;
     let mut new = trial.doc.feature(made).cloned().ok_or_else(|| bad(cmd, format!("`{cid}` did not make a feature")))?;
     if let (FeatureKind::Emboss { face: Some(before), .. }, FeatureKind::Emboss { face: Some(after), .. }) = (&old.kind, &mut new.kind)