Channel-neutral notifications: neutral event, dumb mail channel, Notify node, completer capture

Rework Orchestra notifications into a channel-neutral concern, fully separate from the audit log, and add a visible Notify node plus task-completer capture.

• Kernel: a neutral OrchestraNotificationEvent (recipients, type, params, instance, token, context), a NotificationAudience resolver, and an OrchestraNotification emitter that stamps each event with the instance variables and current node. The audit event is never used to notify.
• orchestra_mail becomes a dumb default channel: one subscriber, one hook_mail template per type, no triggers, no recipient resolution, no inbox dependency.
• Dispatcher modules named *_notification own the trigger and audience: orchestra_interaction_notification (park-on-arrival link) now emits the neutral event, and a new orchestra_inbox_notification emits task assigned, reassigned and reminder off a dedicated non-audit WorkItemEvent and TaskTimedOutEvent. orchestra_interaction_mail and InteractionNotificationEvent are removed (folded into orchestra_mail and the neutral event).
• New orchestra_notification submodule: a Notify workflow node, a visible step the flow routes to conditionally. It renders nothing, resolves its recipients with the reused assignment-row widget, dispatches the event, then advances.
• Task completer capture: a completer variable on the human-task config and an optional actor on complete(), passed by the inbox and the task-interaction resumer, so a later step can address the validator.
• The submission-validation example closes with a Notify node that mails the original poster and the validator once the request is processed. ECA with Easy Email can replace orchestra_mail by subscribing to the same event.

Tests green (kernel and functional), phpcs/phpstan/cspell clean. MR to follow.