<exception> variable should be available at all levels of the stack

Currently when an exception occurs, we inject an `` variable into the top stack frame. This means if you navigate up the stack (eg. if the exception occurred inside the `assert` implementation but you want to view the code that contained the condition) it's not longer visible.

It'd be better if this variable was "global" while you were paused on the exception so you can see it when at any level of the stack.

No files or tests are named in the issue. Start by locating the debugger code that injects the variable into the top stack frame, then trace how variables are exposed when navigating between frames. Done means the variable remains available at every stack level while paused on the exception, with debugger coverage for that behavior.

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