Keep wk_lock held during step commands to prevent worker ping-pong

When multiple independent workers share the well-known lock, releasing
it on step/next/finish allowed a sibling worker to grab the lock before
the stepping worker could re-enter its subsession. This caused the user
to need 2 next commands to actually advance — the first one would
inadvertently drive the other worker.

Only release wk_lock on :continue, which is expected to run for an
extended period. Step commands hold the lock so the same worker
immediately re-enters without yielding.

Author: st0012

lib/debug/session.rb

@@ -1806,7 +1806,9 @@ def get_thread_client th = Thread.current
         DEBUGGER__.debug{ "Leave subsession" }
         bp_sync_publish  # publish breakpoint changes to other processes
         @process_group.unlock
-        @process_group.unlock_wk_lock
+        # Keep wk_lock held during step commands so the same worker
+        # re-enters the subsession without yielding to a sibling.
+        @process_group.unlock_wk_lock if type == :continue
         restart_all_threads
       else
         DEBUGGER__.debug{ "Leave subsession (nested #{@subsession_stack.size})" }