Exchange storage key DKG artifacts over Iroh

[huitseeker]

Stacked on #2433.

This replaces manual public file exchange with one shared Iroh document. One operator starts dkg board from the genesis file, threshold, and epoch, then sends the private ticket to each validator through the trusted bootstrap channel. Each operator runs dkg run with that ticket, the genesis file, its signing key, a private work directory, and an output directory.

The runners exchange signed public artifacts, resume after restarts, and write local storage key bundles. Operators compare the shared public output, keep each secret share private, and start validators only after every runner succeeds. An opt-in Compose check runs the full three-validator flow.

Changelog

[[entry]]
scope       = "validator"
impact      = "added"
description = "Adds an Iroh bulletin board for exchanging storage key DKG artifacts."

[Mirko-von-Leipzig]

I think broadly speaking this would benefit from an explanation/goal and having the code model that.

[Mirko-von-Leipzig]

I'll need to dig into the actual DKG ceremony in more detail first; I'd also like to know what alternatives there are to a central board, or how this would operate for a live network key rotation. I imagine we would want something like this to share txs for new incoming validators etc.

This is beyond the purvey of this PR, but I need to just wrap my head around things.