Hold wk_lock for entire worker session, not just step commands

The previous fix only held the lock during step commands, but continue
between breakpoints had the same ping-pong problem — another worker
could grab the lock before the current one hit its next breakpoint.

Now the wk_lock is never released in leave_subsession. Each worker
keeps exclusive debugger access for its entire lifetime. Other workers
queue up and get their turn when the current one exits. The kernel
releases flock automatically on process exit.

Author: st0012

lib/debug/session.rb

@@ -1806,9 +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
-        # 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
+        # Never release wk_lock here — the current worker keeps exclusive
+        # debugger access until its session ends.  Other workers queue up.
+        # The kernel releases flock automatically on process exit.
         restart_all_threads
       else
         DEBUGGER__.debug{ "Leave subsession (nested #{@subsession_stack.size})" }