[cryptography/bls12381/dkg] Support Fault Model Migration

[patrick-ogrady]

Related: #4592

Our DKG currently assumes that the Fault config stays static across reshares. We should add support for changing Fault (say if migrating to Multimmit).

[patrick-ogrady]

Implementation notes:

The previous sharing is already authoritative for the old round: it records both total() and required(). Fault-model migration therefore should not require retaining or reconstructing the old Faults type. Use the new model consistently for the new round, and derive old-round limits from the previous sharing.

Suggested contract:

• Add a model-independent Sharing::max_faults() helper, computed as total - required after validating required <= total.
• Info::new::<CurrentM> should validate the previous output structurally: its total matches its player set, its threshold is in range, and enough configured dealers exist to meet that threshold.
• Compute required commitments as max(current dealer quorum, previous.public().required()).
• For Reveal::V1 during a reshare, derive the old contributor fault budget from previous.public().max_faults(). The new polynomial degree and maximum reveals remain governed by CurrentM.
• Keep the internal assertion that interpolated reshare commitments preserve the previous group public key.

Regression coverage should use seven participants, where the models have distinct nonzero fault budgets:

1. N3f1 -> N5f1: threshold 5 -> 6. This proves the stricter current quorum raises the required commitment count; the reveal threshold is 6 - 2 = 4.
2. N5f1 -> N3f1: threshold 6 -> 5. This proves the previous recovery threshold still requires six commitments even though the current dealer quorum is five; the reveal threshold is 6 - 1 = 5.

Each test should run a complete honest reshare, exercise both observer and player finalization, verify the new threshold and finalized private share, and confirm that the group public key is preserved. A local prototype of this shape passed both focused tests; no code from it was retained.