[springmin]

Adds the host seam for new-mail system notifications, extracted from #85.

• Drains the existing notification outbox instead of formatting user-visible text in shared code.
• Applies the Important / All / tabs setting: pending AI routing is deferred, out-of-scope rows are suppressed, and rows are claimed before dispatch and marked delivered afterwards.
• Hands the host raw sender, subject and thread/message ids, so each platform translates the notification and attaches its own action (slot type, wantAgent, etc.). The default publisher is a no-op.
• The scope logic has unit tests, which run on the desktop CI target.

No host implementation is included here. The OpenHarmony publisher (translated text, slot type, wantAgent) will follow in #85 once this lands; notifications are currently removed from the OHOS host as requested.

The dispatcher runs on the existing core event listener: once at startup and after each MailNew event.