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 <noreply@anthropic.com>

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  |  \