Merge Watch death-notification system into swactor-std, reducing core
surface. WatchRegistry now lives alongside MonitorRegistry in StdExtension,
with CtxWatching and RuntimeWatching extension traits for the public API.
Phase 5b removed from tick_once; notifications delivered via on_actor_death
in phase 7. Core shrinks from 2,636 to 2,440 lines (-196, -7.4%).
Authored by Claude, lovingly guided by Zachery Aaron Shores-Chmielewski
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