Fix error when lambdas return symbols #27
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Lambda frames reuse argv[0] throughout execution: initially for the lambda form, then for return values during tail-recursive execution. When a lambda body completes, its return value is copied to argv[0] (vm-sched.c:270). For tail recursion, this is correct: when the return is a form like (count (- n 1)), vm_eval_expr evaluates it to continue recursion. For final values, vm_eval_expr should not be called. When the return is a symbol like 'DONE, vm_eval_expr tries to look it up as a procedure name, fails, and raises VM_ERROR_BYTECODE. The fix checks if the lambda frame is complete (VM_EVAL_COMPLETED_ALL) before calling vm_eval_expr. Complete frames don't need re-evaluation, while incomplete frames continue tail recursion normally.
previous commit. The tests validate that lambda return values work correctly for both non-recursive and tail-recursive scenarios, including mutual recursion.
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Lambdas returning symbol values (like 'DONE or 'SUCCESS) were causing crashes with VM_ERROR_BYTECODE. The VM was attempting to evaluate the returned symbol as a procedure name, which failed.
This fix distinguishes between intermediate tail-recursive calls (which need evaluation to continue recursion) and final return values (which should not be re-evaluated). This preserves tail call optimization while preventing the error.