dev: add integration tests for the moved multisig examples

Context

The multisig examples (MultisigSignatureMintBurn, MultisigSignatureTreasury, MultisigProposalTreasury) were moved out of src/multisig/examples/ into test/integration/_mocks/ and converted from modules (+ test mocks) into self-contained top-level contracts. This keeps them as usage examples while positioning them to be exercised as integration-test fixtures — they compose production modules into one deployable contract, which is exactly what test/integration/ is for.

Their previous unit tests, simulators, and mocks were removed in the same change. No integration tests have been added yet — that is the purpose of this issue.

Moved contracts (point of reference)

• contracts/test/integration/_mocks/MultisigSignatureMintBurn.compact — signature-authorized mint/burn of a native shielded token (3 signers, threshold 2).
• contracts/test/integration/_mocks/MultisigSignatureTreasury.compact — signature-authorized single-tx spend from a stateless shielded treasury.
• contracts/test/integration/_mocks/MultisigProposalTreasury.compact — caller-authorized on-chain proposal governance over a shielded treasury.

Task

Add integration coverage for the three, mirroring the existing test/integration/specs/nativeShieldedToken/ suite:

• Wire each contract into the integration compile step (compact:integration in contracts/package.json).
• Add fixtures under test/integration/fixtures/ (signer commitments, salts, coins, recipients).
• Add specs under test/integration/specs/ covering the happy paths (deposit / mint / burn / execute / propose→approve→execute) plus key failure modes (non-signer, below-threshold, wrong token color, replay).
• Decide unit vs network suite — the nativeShieldedToken/ subtree runs under vitest.integration-net.config.ts; the signature examples may want a full-proof path too.

Notes

• ECDSA verification is stubbed in EcdsaSignerManager (see #629), so signature-path tests can only verify membership / threshold / structure, not real signatures, until the primitive lands.
• These examples were relocated from src/multisig/examples/ (originally introduced in #628).

Relates to #628.

Start with contracts/package.json and the existing test/integration/specs/nativeShieldedToken/ suite, then inspect the three contracts in test/integration/_mocks/. Add integration fixtures and specs for the listed happy paths and failure modes, wiring all contracts into compact:integration. Done means the suites compile and cover membership, thresholds, token color, replay, and proposal execution within the chosen test configuration.