Implement Step-Over functionality

This change adds a new "Step Over" button, which is enabled
when a "JSR" (Jump to Subroutine) instruction is encountered
while stepping through execution one instruction at a time.

Pressing "Step Over" will continue running instructions until a
corresponding "RTS" (Return from Subroutine) is encountered.
While stepping over a subroutine, the call stack depth is tracked
in an attempt to halt only when the *correct* RTS is encountered.
In other words, it should skip over other subroutines nested
inside the call you want to step over:

```
     JSR A # Step over here...
     ...
     JSR B
     ...
     RTS   # Returns from 'B', doesn't halt
     ...
     RTS   # Returns from 'A', halts after return
```

Implements feature request #18.

src/main/java/com/loomcom/symon/CpuState.java

@@ -1,3 +1,26 @@
+/*
+ * Copyright (c) 2008-2025 Seth J. Morabito <web@loomcom.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
 package com.loomcom.symon;
 
 import com.loomcom.symon.util.Utils;