dev: trim ShieldedMultiSig preset getters to a deployable set

Context

The ShieldedMultiSig preset's getter surface, combined with its state-changing circuits, sits at or over the local-node deploy block limit. Some getters were already dropped to fit (getProposalRecipient/getProposalAmount/getProposalColor, getReceivedMinusSent), and #673 removed the simulator wrappers that faked them so nothing masquerades as a circuit the preset does not expose.

Problem

• The preset is undeployable / at the ceiling as circuits grow.
• Some getters are trivial derivations that arguably should not be circuits at all: getReceivedMinusSent is just getReceivedTotal - getSentTotal.

Proposal

• Trim the preset getter surface to the minimum needed for on-chain checks; derive the rest off-chain (the TS ledger reader / getProposal).
• Drop getReceivedMinusSent as a circuit in the preset and the ShieldedTreasury module.
• Once removed from the treasury module, drop the receivedMinusSent-equals-balance section in ShieldedTreasury.test.ts (kept in #673 because the module circuit still exists there).

Acceptance

• The preset deploys on the local node within the block limit.
• Derived values are read off-chain, not via faked circuit wrappers.

Origin

Split from #673 review (Andrew's comments on the simulator getter wrappers, https://github.com/OpenZeppelin/compact-contracts/pull/673#discussion_r3592239540 and https://github.com/OpenZeppelin/compact-contracts/pull/673#discussion_r3592280056).

Start with the ShieldedMultiSig preset and the ShieldedTreasury module, then inspect ShieldedTreasury.test.ts and the TS ledger reader's getProposal path. Verify which getters remain needed for on-chain checks and run the preset deployment and treasury tests. Done means the preset deploys within the local-node block limit, derived values are read off-chain, and the obsolete receivedMinusSent assertion is removed.