Stop pointer parameters leaking value-pool slots

Create structure parameters before allocating their representation, then keep pointer references in the call variable's inline slot. Add an 8,000-call regression and document the remaining by-value structure escape limitation.

Co-authored-by: andrew <andrew@aklabs.net>
Co-authored-by: OpenAI Codex (GPT-5) <noreply@openai.com>

include/akbasic/environment.h

@@ -246,6 +246,16 @@ akerr_ErrorContext AKERR_NOIGNORE *akbasic_environment_collect_subscripts(akbasi
  */
 akerr_ErrorContext AKERR_NOIGNORE *akbasic_environment_create(akbasic_Environment *obj, const char *varname, akbasic_Variable **dest);
 
+/**
+ * @brief Create a variable slot without allocating value storage.
+ *
+ * Used when the caller knows the variable's representation before its first
+ * initialization, such as a structure parameter. The caller must initialize
+ * the variable before evaluating it.
+ */
+akerr_ErrorContext AKERR_NOIGNORE *akbasic_environment_create_empty(akbasic_Environment *obj, const char *varname,
+										akbasic_Variable **dest);
+
 /**
  * @brief Resolve a label to the line number it marks.
  * @param obj Scope to search; the parent chain is walked.