feat(pline): Undo option removes the last segment (#352)

U / UNDO (typed or via the prompt button) pops the last placed vertex
without ending the command, repeatably down to and past the start
point. The exit tangent of the remaining last segment is recomputed
from its bulge so a following Arc segment stays tangent-continuous.
New CmdResult::RemoveLiveEntity takes the live polyline out of the
document when Undo drops it below two vertices; the next point
re-creates it under the same undo unit.

Closes #352

Co-Authored-By: Claude Fable 5 <[email protected]>

src/command.rs

@@ -983,6 +983,9 @@ pub enum CmdResult {
         entity: EntityType,
         finish: bool,
     },
+    /// Remove the live entity from the document but keep the command running —
+    /// PLINE's Undo popping back below the two vertices an entity needs.
+    RemoveLiveEntity(Handle),
     /// Suspends command execution, moves it to suspended_cmd, and opens the text editor for the given handle.
     SuspendForTextEdit { handle: Handle },
     /// Requests a standard document-level undo while keeping the command active.