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/mod.rs

@@ -57,6 +57,10 @@ use serde_json::Value;
 use crate::{Result, Session};
 
 pub type Run = fn(&mut Session, &Value) -> Result<Value>;
+/// A command's form for one frame of an interactive gesture: its result and the sketch it
+/// changed, for [`Session::execute`] to show without evaluating the rest of the model (`None`:
+/// evaluate as usual).
+pub type LiveRun = fn(&mut Session, &Value) -> Result<(Value, Option<crate::LiveEdit>)>;
 pub type Enabled = fn(&Session) -> std::result::Result<(), String>;
 
 pub struct CommandSpec {
@@ -74,11 +78,40 @@ pub struct CommandSpec {
     pub enabled: Enabled,
     pub run: Run,
     pub undoable: bool,
+    /// What runs instead of `run` when the command is given `live: true` (a frame of an
+    /// interactive gesture, see [`Session::execute`]).
+    pub live: Option<LiveRun>,
+    /// Reads the evaluated model (the default). A command that reads only the document and the
+    /// session's history runs on a model left stale by a live gesture without evaluating it first.
+    pub reads_model: bool,
 }
 
 impl CommandSpec {
     pub const fn new(id: &'static str, label: &'static str, run: Run) -> Self {
-        CommandSpec { id, label, tab: "", panel: "", icon: "", shortcut: None, params: "", enabled: always, run, undoable: true }
+        CommandSpec {
+            id,
+            label,
+            tab: "",
+            panel: "",
+            icon: "",
+            shortcut: None,
+            params: "",
+            enabled: always,
+            run,
+            undoable: true,
+            live: None,
+            reads_model: true,
+        }
+    }
+    /// Takes `live: true`, run by `run`.
+    pub const fn live(mut self, run: LiveRun) -> Self {
+        self.live = Some(run);
+        self
+    }
+    /// Reads only the document (and history), never the evaluated model.
+    pub const fn doc_only(mut self) -> Self {
+        self.reads_model = false;
+        self
     }
     pub const fn at(mut self, tab: &'static str, panel: &'static str) -> Self {
         self.tab = tab;
@@ -105,6 +138,12 @@ impl CommandSpec {
         self.undoable = false;
         self
     }
+    /// Run inside another command: when it is enabled, its `run` (the caller's command owns the
+    /// undo step and the evaluation).
+    pub(crate) fn run_nested(&self, s: &mut Session, p: &Value) -> Result<Value> {
+        (self.enabled)(s).map_err(|why| crate::EngineError::Disabled(self.id.to_string(), why))?;
+        (self.run)(s, p)
+    }
     pub fn info(&self, s: &Session) -> CommandInfo {
         let e = (self.enabled)(s);
         CommandInfo {