Fix ACPClient weakref accumulation

[miky-rola]

Summary

• Prune dead weakrefs from _live_instances on each new ACPClient init
• Guard atexit.register() with a class flag so it only registers once

Test plan

• test_dead_weakrefs_pruned — dead refs removed on new instance creation
• test_atexit_registered_once — atexit registers exactly once
• test_atexit_cleanup_closes_live_and_clears — cleanup calls close and clears list
• Existing ACP tests pass (8/8)