[NF-06] Add on-chain Guardian Multisig N-of-M threshold signer contract

[devwums]

Summary

Introduces the guardian_multisig Soroban contract that satisfies the Guardian role expected by the Governance contract.

Changes

• contracts/guardian_multisig/src/lib.rs — N-of-M threshold multisig contract with:
  • initialize(owners, threshold) — registers up to 10 owners and a confirmation threshold
  • confirm_veto(governance_contract, proposal_id) — collects owner confirmations and auto-executes Governance::veto once the threshold is reached
  • confirmation_count, owners, hreshold — read-only view functions
  • Full unit test suite covering initialisation, single-owner 1-of-1 path, and threshold validation
• contracts/guardian_multisig/Cargo.toml — crate manifest with soroban-sdk = "20"
• Cargo.toml — registers contracts/guardian_multisig in the workspace members list

closes #81

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