feat: operator-driven retry and error taxonomy for ceremonies

[lomigmegard]

Introduce an in-run error model so a ceremony can recover from transient failures (a loose cable, an absent token) without losing a long-running session, while ensuring procedural failures are never silently re-run.

Error classification on the types:

• Retriability (Transient / Fatal) with BackendError::retriability() and ActionError::retriability(), classified by an exhaustive, wildcard-free match so every future variant must be deliberately classified. Environmental conditions are transient; malformed data, unsupported operations, and terminal device states are fatal. A procedural failure such as a verification mismatch is never retriable.

Transcript taxonomy:

• ErrorClass (environmental / procedural / integrity / abort) recorded on ErrorRecord, so an auditor can tell a failure's nature without parsing the message and abort is distinguishable from failure.
• New StepAttemptFailed { step, attempt, error } fact; reports group attempts under their step.

Runtime retry:

• A transient error pauses the step and prompts the operator to retry or abort, recorded through the existing prompt machinery. A conservative re-executability gate refuses to retry once an attempt has emitted any evidence, which also keeps retried attempts from reusing entropy.

DSL retry: field:

• retry: never and retry: { attempts: N } constrain the retry-by-default; an absent field prompts the operator unlimited times. A zero attempt budget is rejected with a resolver diagnostic.