Copy, cut and paste sketch geometry (Ctrl+C/X/V, context menu) (#1500)

* Copy, cut and paste sketch geometry (Ctrl+C/X/V, context menu)

Sketch geometry had no copy or paste: Ctrl+C / Ctrl+X / Ctrl+V arrive as
clipboard events that nothing handled, and the sketch context menu had no
Copy, Cut or Paste.

- sketch.copy / sketch.cut / sketch.paste: lines, arcs, circles, ellipses,
  splines, conics, points and text. Constraints and dimensions wholly among
  the copied entities come along (each dimension gets its own parameter
  with the same expression); ones that reach outside, fixes and pattern or
  offset relations are dropped. Paste goes centred at `at`, by
  `translate`, or offset from the original (same sketch) / at the same
  coordinates (another sketch, on its plane). One undo step; the pasted
  geometry is selected.
- Ctrl+C / Ctrl+X / Ctrl+V in a sketch (paste at the cursor), and Cut,
  Copy and Paste in the sketch context menu. Outside a sketch Ctrl+C /
  Ctrl+V copy and paste the selected timeline features. A copy also puts a
  line of text on the system clipboard so the next Ctrl+V is delivered.

Fixes #1480

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

* Keep a pasted text's height and angle expressions, drop its place's

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

---------

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

crates/engine/src/lib.rs

@@ -222,6 +222,8 @@ pub struct Session {
     removed_later_cache: std::sync::Mutex<Option<occurrences::RemovedLaterCache>>,
     /// Features copied with `timeline.copy` (pasted by `timeline.paste`).
     pub clipboard: Vec<solidcraft_doc::Feature>,
+    /// Sketch geometry copied with `sketch.copy` (pasted by `sketch.paste`).
+    pub sketch_clipboard: Option<cmd::sketch_clipboard::SketchClip>,
     /// Section Analysis: a view cut by a plane (origin, unit normal; the normal side is hidden).
     pub section: Option<(Vec3, Vec3)>,
     /// Surface analysis shading the model (zebra, draft, curvature map).
@@ -326,6 +328,7 @@ impl Session {
             occurrence_visibility_cache: Default::default(),
             removed_later_cache: Default::default(),
             clipboard: Vec::new(),
+            sketch_clipboard: None,
             section: None,
             analysis: None,
             checkpoints: Vec::new(),
@@ -792,6 +795,7 @@ impl Session {
             occurrence_visibility_cache: Default::default(),
             removed_later_cache: Default::default(),
             clipboard: self.clipboard.clone(),
+            sketch_clipboard: self.sketch_clipboard.clone(),
             section: None,
             analysis: None,
             checkpoints: Vec::new(),