Fix turtle-lib API docs and invariant wording

Agent-Logs-Url: https://github.com/enaut/turtlers/sessions/5fb2f5ed-163f-4c2d-8180-1e96150e7e46

Co-authored-by: enaut <290005+enaut@users.noreply.github.com>

turtle-lib/src/commands.rs

@@ -34,7 +34,8 @@ pub enum TurtleCommand {
 
     // Position
     Goto(Coordinate),
-    /// Heading stored as radians — already converted by the builder.
+    /// Heading stored as internal radians (Y-down render-space convention),
+    /// already converted by the builder from user-facing degrees.
     SetHeading(Radians),
 
     // Visibility