Automated fix by @leftibot.

What changed

Fix #635: Segfault in optimized for-loop when := copies stack-local variable
The optimized for-loop in chaiscript_optimizer.hpp stored the loop counter
as a stack-local int and exposed it to ChaiScript via var(&i), creating a
reference. When the := operator was used inside the loop body (e.g.
ret := i), it copied the raw pointers to this stack variable into the
target's Data. When the function returned via exception unwinding, the
stack frame was destroyed, leaving dangling pointers in the return value.
Accessing these pointers in the caller (e.g. via string interpolation)
caused a segfault. The fix heap-allocates the loop counter via
std::make_shared, so the data survives scope exit through
reference counting.
Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

Files

 .../chaiscript/language/chaiscript_optimizer.hpp    |  6 +++---
 unittests/future_assign.chai                        | 21 +++++++++++++++++++++
 2 files changed, 24 insertions(+), 3 deletions(-)

Closes #635

Triggered by @lefticus.