Feat: JOIN command — merge collinear Lines or co-circular Arcs

Selection-gathering mode: select objects, press Enter to apply.
- Lines: checks collinearity + no-gap, merges into single line span
- Arcs: checks same center/radius + contiguous angles, merges to Arc or Circle
- Uses CmdResult::JoinEntities; geometry computed in cmd_result from document

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/cmd_result.rs

@@ -371,6 +371,42 @@ impl H7CAD {
                 self.restore_pre_cmd_tangent();
                 self.command_line.push_output(&msg);
             }
+            CmdResult::JoinEntities(handles) => {
+                use crate::modules::home::modify::join::join_entities;
+                let pairs: Vec<_> = handles.iter()
+                    .filter_map(|&h| self.tabs[i].scene.document.get_entity(h).map(|e| (h, e)))
+                    .collect();
+                match join_entities(&pairs) {
+                    Some((to_remove, merged)) => {
+                        let label = self.history_label_from_active_cmd(i, "JOIN");
+                        self.push_undo_snapshot(i, label);
+                        self.tabs[i].scene.erase_entities(&to_remove);
+                        let count_in = to_remove.len();
+                        let count_out = merged.len();
+                        for e in merged {
+                            self.tabs[i].scene.add_entity(e);
+                        }
+                        self.tabs[i].dirty = true;
+                        self.tabs[i].scene.clear_preview_wire();
+                        self.tabs[i].active_cmd = None;
+                        self.tabs[i].snap_result = None;
+                        self.restore_pre_cmd_tangent();
+                        self.command_line.push_output(
+                            &format!("JOIN: {count_in} object(s) joined into {count_out}.")
+                        );
+                        self.refresh_properties();
+                    }
+                    None => {
+                        self.tabs[i].active_cmd = None;
+                        self.tabs[i].snap_result = None;
+                        self.tabs[i].scene.clear_preview_wire();
+                        self.restore_pre_cmd_tangent();
+                        self.command_line.push_error(
+                            "JOIN: objects are not collinear/co-circular or have gaps."
+                        );
+                    }
+                }
+            }
             CmdResult::BreakEntity { handle, p1, p2 } => {
                 use crate::modules::home::modify::break_cmd::break_entity;
                 let replacement = self.tabs[i].scene.document

src/app/commands.rs

@@ -970,7 +970,14 @@ impl H7CAD {
                 }
             }
 
-            // ── Break ────────────────────────────────────────────────────────
+            // ── Break / Join ─────────────────────────────────────────────────
+            "JOIN"|"J" => {
+                use crate::modules::home::modify::join::JoinCommand;
+                let cmd = JoinCommand::new();
+                self.command_line.push_info(&cmd.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(cmd));
+            }
+
             "BREAK"|"BR" => {
                 use crate::modules::home::modify::break_cmd::BreakInteractiveCommand;
                 let cmd = BreakInteractiveCommand::new();
@@ -1003,7 +1010,7 @@ impl H7CAD {
             "HELP"|"?" => {
                 self.command_line.push_output(
                     "Draw: LINE CIRCLE ARC PLINE RECT POLY POINT ELLIPSE SPLINE RAY XLINE HATCH  |  \
-                     Modify: MOVE COPY ROTATE SCALE MIRROR ERASE OFFSET EXTEND FILLET CHAMFER STRETCH EXPLODE TRIM BREAK  |  \
+                     Modify: MOVE COPY ROTATE SCALE MIRROR ERASE OFFSET EXTEND FILLET CHAMFER STRETCH EXPLODE TRIM BREAK JOIN  |  \
                      Array: ARRAY ARRAYRECT ARRAYPOLAR ARRAYPATH  |  \
                      Text: TEXT MTEXT LEADER MLEADER  |  \
                      Dimension: DIMLINEAR DIMANGULAR DIMRADIUS  |  \