Add DKLS23 soft-spoken presignatures

[toufic0710]

Description

Resolves #255

Previously, all final signing logic lived in pkg/mpc/signatures/ecdsa/dkls23/signing_softspoken/rounds.go, inside Round5: it computed the message-independent values, hashed the message, and returned a partial signature.

This PR splits that logic across the following files:

• pkg/mpc/signatures/ecdsa/dkls23/presignature.go Defines the shared dkls23.PreSignature type, its CBOR encoding/ decoding, and Finalise, which turns a presignature into a partial signature once the message is known.

• pkg/mpc/signatures/ecdsa/dkls23/signing_softspoken/ presignature.go Contains the soft-spoken PreSign logic extracted from the old Round5. This is the message-independent part of round 5.

• pkg/mpc/signatures/ecdsa/dkls23/signing_softspoken/rounds.go Keeps the existing Round5 API, now implemented as PreSign followed by Finalise.

• pkg/mpc/signatures/ecdsa/dkls23/presign_softspoken/runner.go Adds a high-level runner that executes the soft-spoken signing rounds up to PreSign, producing a presignature before the message is known.

• Tests cover both the round-level split and the presignature runner path.

We don't need additive key derivation or "raw signing", so we did not implemented rerandomization.

Pull Request Checklist

Scope

• Summary and description are provided.
• Changes are focused and scoped to one purpose.

Testing (select all that apply)

• Unit tests
• Property tests
• Test vectors tests
• Benchmarks
• Not run (explain why)

Documentation / Spec (if relevant)

• Updated/added docs or comments
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Notes

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[toufic0710]

Open questions:

• presign_softspoken.NewRunner mirrors most of signing_softspoken.NewRunner and differs mainly at the final step (PreSign instead of Round5). Would you prefer the runner logic to be factored to reduce duplication, or is the explicit separate runner acceptable?
• would you like BBOT presigning to be inside this PR, or handled in a follow-up PR?