GH-126491: GC: Mark objects reachable from roots before doing cycle collection (GH-127110)

* Mark almost all reachable objects before doing collection phase

* Add stats for objects marked

* Visit new frames before each increment

* Update docs

* Clearer calculation of work to do.

Include/internal/pycore_runtime_init.h

@@ -137,6 +137,7 @@ extern PyTypeObject _PyExc_MemoryError;
                 { .threshold = 0, }, \
             }, \
             .work_to_do = -5000, \
+            .phase = GC_PHASE_MARK, \
         }, \
         .qsbr = { \
             .wr_seq = QSBR_INITIAL, \