feat(sdk): expose subaccount commitments (#892)

Adds local SDK helpers on subaccounts(dappName): partialCommitment() for hash(identity_key, dappName), and commitment(nonce) for hash(partialCommitment, nonce). These are computed entirely in TypeScript and do not call the anonymizer contract or discovery service.

Validation:
- npx vitest run --coverage.enabled=false tests/internal/sub-accounts.test.ts
- npm run build
- git diff --check

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