Ractor: mark a terminated Ractor's join value through one function

rb_ractor_mark_in_flight_for_single_objspace marked r->sync.legacy and
nothing else once the pin machinery went away, which is what
rb_ractor_mark_terminated_join_value next to it already does -- two functions
over the same field, and the surviving one names it correctly.  Call that one
from the single-objspace branch and drop the other.

It also pins, which the plain rb_gc_mark did not.  sync.legacy is a C-struct
slot with no compaction update hook anywhere, so pinning is the treatment the
field needs; the comment on rb_ractor_mark_terminated_join_value says so.

The name had stopped describing the function: "in flight" meant the sending
basket and the pinned copy snapshot it used to mark, and both are gone.  The
word is also the GC-cycle sense elsewhere in this code, which is why the
wrapper it sat next to is now rb_gc_rest.

Only the ractor.c translation unit used it -- ractor.c includes
ractor_sync.c -- so the ractor_core.h declaration goes too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ractor.c

@@ -306,11 +306,11 @@ ractor_mark(void *ptr)
      * both the set and zombie_objspaces (orphan-merged) this marker is its only cover. */
     rb_gc_mark(r->sync.default_port_value);
     /* A single-objspace impl (mmtk) has no zombie_objspaces and no pin/shref bits, so
-     * the root scan cannot reach a terminated Ractor's legacy value, queue or in-flight
-     * payloads; and no shref rule forbids following them from the wrapper. */
+     * the root scan cannot reach a terminated Ractor's queue, in-flight payloads or
+     * join value; and no shref rule forbids following them from the wrapper. */
     if (!rb_gc_multi_objspace_p()) {
         ractor_mark_unshareable_parts(r);
-        rb_ractor_mark_in_flight_for_single_objspace(r);
+        rb_ractor_mark_terminated_join_value(r);
     }
 }
 

ractor_core.h

@@ -159,7 +159,6 @@ struct rb_ractor_struct {
 /* Mark the GC roots held in Ractor r's C structs (from the root scan in gc.c). */
 void rb_ractor_mark_local_roots(rb_ractor_t *r);
 void rb_ractor_mark_terminated_join_value(rb_ractor_t *r);
-void rb_ractor_mark_in_flight_for_single_objspace(rb_ractor_t *r);
 
 /* Move src's registered_marks to dst and leave src empty (on join or when an orphan
  * is absorbed).  An absorb can run during a GC sweep, so the implementation uses raw

ractor_sync.c

@@ -825,15 +825,6 @@ ractor_sync_mark(rb_ractor_t *r)
     }
 }
 
-/* A single-objspace impl (mmtk) has no pin or shref bits and no zombie_objspaces, so
- * plain marking from the wrapper keeps these alive; the default GC covers the same set
- * with its pins and its zombie scan. */
-void
-rb_ractor_mark_in_flight_for_single_objspace(rb_ractor_t *r)
-{
-    rb_gc_mark(r->sync.legacy);
-}
-
 static int
 ractor_sync_free_ports_i(st_data_t _key, st_data_t val, st_data_t _args)
 {