refactor: isolate iroh bulletin board transport

[huitseeker]

Summary

This PR builds on #2434 and narrows the part of the validator that knows about Iroh.

• Adds separate board roles for the coordinator and participants. A participant can publish only to its own slots.
• Keeps the shared slot rules in one transport-neutral module.
• Moves the Iroh upload protocol and saved state code into private modules.
• Makes the ceremony runner accept a constructed board instead of choosing a transport.
• Adds a memory adapter for contract tests and full ceremony tests without Iroh.

The command flow and board file formats do not change. Ticket text, upload messages, and saved board data keep the same format.

Changelog

[[entry]]
scope       = "validator"
impact      = "changed"
description = "Isolated the storage key DKG bulletin board from its Iroh transport."