feat(sdk-lib-mpc): EddsaRetrofitData type + DKG retrofit constructor + getFirstMessage routing

[bitgo-ai-agent-dev]

What

• Add optional `retrofitData?: EddsaRetrofitData` 4th parameter to the `DKG`
  constructor in `modules/sdk-lib-mpc/src/tss/eddsa-mps/dkg.ts`.
• Store `retrofitData` as a private field on the instance.
• In `getFirstMessage()`, branch on `this.retrofitData`: when set, call
  `wasm.ed25519_dkg_round0_import` with the party's clamped scalar,
  aggregate public key, and chain code; otherwise fall through to the
  existing `ed25519_dkg_round0_process` path.
• Export `EddsaRetrofitData` as a named type from `eddsa-mps/index.ts` so
  callers can import it directly without going through the `MPSTypes` namespace.
• Bump `@bitgo/wasm-mps` from `1.10.0` → `1.12.0` (exports
  `ed25519_dkg_round0_import` with a proper TypeScript signature — no
  `as any` cast needed) and update `yarn.lock` accordingly.
• Extend `generateEdDsaDKGKeyShares` in `util.ts` to accept per-party
  `retrofitData` params and forward them to the `DKG` constructor.
• Add three retrofit DKG tests in `test/unit/tss/eddsa/dkg.ts`:
  • routing test (all parties agree on public key after retrofit DKG)
  • differentiation test (retrofit vs fresh DKG produce distinct keys)
  • determinism test (same `retrofitData` inputs produce same public key)

Why

• Parties migrating from MPCv1 (Zengo) to MPCv2 (Silence Labs MPS) already
  hold a valid EdDSA scalar (`pShare.u`). Generating fresh key material would
  produce a new public key, breaking wallet continuity.
• `ed25519_dkg_round0_import` accepts the existing scalar and seeds a
  retrofit DKG ceremony that produces MPCv2 shares for the same aggregate
  public key, enabling transparent migration without user action.
• Rounds 1 and 2 (`handleIncomingMessages`) are unchanged — both paths
  produce identical `dkgStateBytes` format, so no downstream code changes.

Test plan

• `yarn workspace @bitgo/sdk-lib-mpc test` — all existing + new DKG tests pass.
• TypeScript compiles cleanly in `modules/sdk-lib-mpc`.

Ticket: WCI-1261

[linear-code]

WCI-1261