feat(CHAIN-3446): add signer enumeration to SystemConfigGlobal

[leopoldjoy]

Summary

Adds getRegisteredSigners() to SystemConfigGlobal, enabling callers to read the full set of registered signers in a single view call. This is a prerequisite for the prover registrar service (CHAIN-3449), which needs to enumerate on-chain signers each poll cycle to detect orphaned entries for deregistration.

Changes

• Import and use Solady EnumerableSetLib.AddressSet (already used elsewhere in the multiproof contracts)
• Add EnumerableSetLib.AddressSet internal _registeredSigners storage (appended to end of layout — safe for upgrades)
• registerSigner: call _registeredSigners.add() alongside the existing signerPCR0 mapping update
• deregisterSigner: call _registeredSigners.remove() alongside the existing mapping delete
• New getRegisteredSigners() external view returns (address[]) — no event scanning required, always authoritative
• DevSystemConfigGlobal.addDevSigner: also maintains the set so tests reflect production behaviour
• Version bumped to 0.2.0
• 6 new tests covering empty set, register, deregister, multiple signers, mixed operations, and idempotent deregistration

Test plan

• forge test --match-contract SystemConfigGlobalTest — 36 tests pass

[linear]

CHAIN-3446 [Contracts] Add signer enumeration to SystemConfigGlobal

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1