feat(component-failover-adoption): validate worker thread isolation, fix rejoin bugs found by E2E

Confirm connector thread is isolated from worker_process_update() (main
thread) and resource_sync_thread() (own thread) via code read plus a real
pthread concurrency test. Add unit and live 3-node E2E coverage for
candidate/reconnect behavior, all-candidates-unreachable diagnostics, and
liveness cadence under concurrent reconnect.

The E2E run surfaced two production bugs the 2.1/2.2 unit tests missed:
xcecli_start() resets the reconnect callback to NULL after it runs, so
registering it before xcecli_start() silently no-ops the whole rejoin
mechanism; and a successful worker join returns its assigned slot index
as reply.status (any rc >= 0), not always exactly 0, so checking_connection()'s
rc == 0 success check never fired on a real non-zero-slot join. Both fixed,
plus per-candidate names added to the all-unreachable failure log.

Root-check bypass needed for the E2E run is gated at compile time via the
existing VIRTUAL_CLUSTER flag, not a runtime env var, so the real xcecd
binary's root requirement has zero bypass surface.

src/worker/testsuite/meson.build

@@ -13,7 +13,10 @@ worker_test_deps = [xcecd_worker_deps, unity_dep]
 
 controller_connector_test = executable('controller_connector_test',
                                        'test_controller_connector.c',
-                                       c_args: ['-DDBUS_MANAGER'],
+                                       c_args: ['-DDBUS_MANAGER',
+                                                '-DXCE_CONTROLLER_CONNECTOR_C_PATH="'
+                                                + (meson.current_source_dir() / '..' / 'src' / 'controller_connector.c')
+                                                + '"'],
                                        include_directories: [worker_test_incdir],
                                        dependencies: worker_test_deps,
                                        install: false)