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>

ROADMAP.md

@@ -44,7 +44,7 @@ PCB / Package, Render, Animation, Simulation, Manufacture/CAM, Drawings.
 | # | Milestone | Status | Hours left |
 |---|---|---|---|
 | M0 | Vertical slice | done | — |
-| M1 | Sketch depth: command coverage implemented — projection/intersect/include as linked geometry (auto-project on faces, lost-reference warnings, break link), full SKETCH create/modify set (slots, polygons, tangent arcs/circles, ellipse, fit/control splines, conic, text, trim/extend/break, fillet/chamfer, offset, mirror, patterns, move/scale, blend curve, centerline), constraints incl. curvature and polygon with over-constraint refusal, driven/arc-length/linear-diameter dimensions, AutoConstrain, DXF/SVG insert, 3D sketch curves (Project To Surface, Intersection Curve, Isoparametric Curve, Spun Profile), exact free-form profiles (B-spline/NURBS edges, any spline degree, curves cut by crossings), sweeps and pipes along 3D sketch curves, canvases and decals, curvature combs. Exact free-form Extend and certified associative offsets now preserve geometry and IDs through edits, deletion and reopen. Occurrence-frame 3D wires now store explicit coordinate spaces and retain points and analytic fits through moves, refresh, undo and reopen. Remaining workflow repairs: mixed offset junctions, legacy natural-spline seams, and damaged legacy wire links lacking frame provenance | in progress | 4 |
+| M1 | Sketch depth: command coverage implemented — projection/intersect/include as linked geometry (auto-project on faces, lost-reference warnings, break link), full SKETCH create/modify set (slots, polygons, tangent arcs/circles, ellipse, fit/control splines, conic, text, trim/extend/break, fillet/chamfer, offset, mirror, patterns, move/scale, blend curve, centerline), constraints incl. curvature and polygon with over-constraint refusal, driven/arc-length/linear-diameter dimensions, AutoConstrain, DXF/SVG insert, 3D sketch curves (Project To Surface, Intersection Curve, Isoparametric Curve, Spun Profile), exact free-form profiles (B-spline/NURBS edges, any spline degree, curves cut by crossings), sweeps and pipes along 3D sketch curves, canvases and decals, curvature combs. Exact free-form Extend and certified associative offsets now preserve geometry and IDs through edits, deletion and reopen. Occurrence-frame 3D wires now store explicit coordinate spaces and retain points and analytic fits through moves, refresh, undo and reopen. Constructive sketch solving (Owen 1991, Fudos & Hoffmann 1997) ahead of the numerical solver: points and rigid clusters are placed on the branch the sketch had, so dimension edits and drags keep their solution; edits that would turn a sketch over are refused; drags re-solve only the sketch per frame. Remaining workflow repairs: mixed offset junctions, legacy natural-spline seams, and damaged legacy wire links lacking frame provenance | in progress | 4 |
 | M2 | Robust modelling: coplanar booleans in the kernel, holes (placed on faces and on sketch points), threads (cosmetic, then modelled), patterns, mirror, shell, draft, split, construction geometry | in progress | 16 |
 | M3 | General fillets and chamfers (curved edges, chains, vertex blends); saddle rolling-ball surfaces now use continuous seam sections and arc-length fitting with shared contact boundaries and bounded accuracy checks | in progress | 30 |
 | M4 | Files: STEP import (done — [docs/step-import.md](docs/step-import.md)), 3MF export and 3MF/STL import as mesh bodies (done), constant-section parametric mesh reconstruction, DXF sketches, OBJ import | in progress | 6 |