fix(worker): dbus_workers_update() starves workers after first DONE/EXITED (bug-1589)

Every early-exit branch in the worker_list scan either re-tested the same
node forever (bare `continue`) or returned out of the whole function on the
first DONE/EXITED worker, so job array elements past the head of the list
never got their on_job_start report. Capture `next` before touching each
node and always advance to it. Affects xcecd_virtual and real xcecd alike
(same DBUS_MANAGER-gated function).

Also records a roadmap follow-up (xcecd_virtual concurrent-job support,
MXJ > 1) discovered while investigating this bug.

src/worker/testsuite/meson.build

@@ -58,3 +58,22 @@ test('worker xcecd_startup_order test', xcecd_startup_order_test,
     suite: 'worker',
     timeout: 60,
     env: ['ASAN_OPTIONS=detect_leaks=0'])
+
+# bug-1589: dbus_workers_update() (xcecd_dbus_manager.c) worker-list
+# traversal regression test -- see test_dbus_workers_update.c's header
+# comment for the full root-cause trace. Same -DDBUS_MANAGER
+# -DVIRTUAL_CLUSTER flag pair the real `xcecd_virtual` executable uses
+# (worker/meson.build); DBUS_MANAGER has no effect on xcecd_dbus_manager.c
+# itself (no #ifdef DBUS_MANAGER branches there) but is included to match
+# production's exact build flags.
+dbus_workers_update_test = executable('dbus_workers_update_test',
+                                      'test_dbus_workers_update.c',
+                                      c_args: ['-DDBUS_MANAGER', '-DVIRTUAL_CLUSTER'],
+                                      include_directories: [worker_test_incdir],
+                                      dependencies: worker_test_deps,
+                                      install: false)
+
+test('worker dbus_workers_update test', dbus_workers_update_test,
+    suite: 'worker',
+    timeout: 60,
+    env: ['ASAN_OPTIONS=detect_leaks=0'])