dev: restructure multisig module layout and consolidate duplicated preset logic

Summary

Reorganize multisig/ modules into concern-based subdirectories, retire the
duplicated signer module, and extract shared logic so presets become thin,
composable wirings instead of monolithic contracts.

Motivation

• No contract-to-contract calls on Midnight → capabilities must be composed into
  one contract. The current presets can't compose: each is a top-level contract
  with its own constructor, and V2/V3 redefine the same structs/circuits.
• Signer.compact and SignerManager.compact are one module in two generations
  (identical state; Signer adds init guards). Keeping both is leftover migration.
• ShieldedMultiSig / V2 / V3 imply a version lineage, but they are three
  independent designs (proposal / signature / token mint-burn).
• ECDSA-commitment verification is copy-pasted verbatim across V2 and V3; V1
  inlines proposal-approval tracking that belongs in ProposalManager.

Proposed change

Layout (modules grouped by concern; test/mocks/ mirrors it):

multisig/
├── signer/      SignerManager  (+ SignatureVerifier)
├── proposal/    ProposalManager
├── treasury/    ShieldedTreasury / ShieldedTreasuryStateless / UnshieldedTreasury
├── forwarder/   ForwarderPrivate / ForwarderShielded / ForwarderUnshielded
└── presets/     …  (presets/forwarder/ unchanged)

• Retire legacy SignerManager; rename Signer → SignerManager (hardened
  content). Migrate imports, mocks, and Signer: → SignerManager: error strings.
• Extract a SignatureVerifier module from the V2/V3 duplication.
• Fold V1's approval tracking into ProposalManager.
• Rename presets by behavior: ShieldedProposalMultiSig (V1),
  ShieldedSignatureMultiSig (V2), ShieldedTokenMultiSig (V3). Standardize
  MultiSig casing.