Issue #3607272: Channel-neutral notifications, dumb mail channel, Notify node, completer capture

Implements #3607272. Neutral OrchestraNotificationEvent + emitter + audience resolver in the kernel; orchestra_mail becomes a dumb channel; *_notification dispatcher modules (interaction + new orchestra_inbox_notification) off dedicated non-audit events; new orchestra_notification submodule with a visible Notify node; task completer capture; example wired to mail poster+validator once processed. Audit event never drives notifications. ECA+Easy Email can replace orchestra_mail. Tests green; phpcs/phpstan/cspell clean.