feat: add _Atomic gc-predicate fields to xce_scheduler_t (phase 6 task 6-3)

src/scheduler/include/scheduler.h

@@ -4,6 +4,7 @@
 
 #include <pthread.h>
 #include <rtti.h>
+#include <stdatomic.h>
 #include <stdint.h>
 #include <time.h>
 
@@ -83,6 +84,9 @@ typedef struct {
     cbuf_t            wbuf;      /* action to trigger scheduling */
     cbuf_t            jbuf;      /* job picked for allocation */
     cbuf_t            decisions; /* scheduler decisions */
+    _Atomic int       session_active;         /* 1 during scheduling session, 0 between */
+    _Atomic int       eval_in_progress;       /* 1 while eval_select loop runs */
+    _Atomic int       staging_merge_complete; /* 1 after staging merge completes */
 } xce_scheduler_t;
 
 extern int  sched_init(xce_scheduler_t*, xce_client_config_t*,