Carry file attachments on OrchestraNotificationEvent and honor them in the mail channels

Problem/Motivation

OrchestraNotificationEvent is the channel-neutral contract for notifications: resolved recipients plus structured params, delivered by whichever channel is enabled. There is currently no way for a dispatcher to attach a file (a generated PDF, an ICS invite, a receipt), so a channel cannot deliver one.

The concrete driver is the Yoyaku ticket submodule (yoyaku #3611638): on order confirmation it generates a PDF ticket that should ride along on the existing confirmation email rather than forcing a second, separate email. An order can hold several tickets (one per booked place), so a notification must be able to carry multiple attachments, one per ticket.

Proposed resolution

Add a neutral attachments payload to OrchestraNotificationEvent and have the shipped channels honor it.

1. New NotificationAttachment value object (@api): filename, mimeType, and one of content (raw bytes) or uri (a stream-wrapper URI), so a producer can pass either an in-memory blob or a file.
2. New trailing array $attachments = [] constructor argument on OrchestraNotificationEvent, a list of NotificationAttachment. Being a list, one notification can carry many files (one per ticket). It is appended last, so existing dispatchers are unaffected.
3. orchestra_mail: pass the attachments through so hook_mail sets the message attachments (the de-facto convention honored by Symfony Mailer and Mime Mail). The default PHP mail backend does not send attachments; this is a mail-system capability, not an Orchestra one.
4. orchestra_easy_email: map the attachments onto the Easy Email entity before send.

Remaining tasks

• Implement the value object and the event change.
• Honor attachments in both channels.
• Unit and kernel coverage for both channels: attachment present, empty default, uri versus content, and multiple attachments.

API changes

• OrchestraNotificationEvent::__construct() gains a trailing array $attachments = [] argument (backward compatible).
• New @api NotificationAttachment value object.