H2: Replace (List<Coin>, List<Key>) tuple with SigningCoin record

[DavidGershony]

Summary

Replace the unsafe (List<Coin>?, List<Key>) tuple return type with a strongly-typed SigningCoin record that pairs each coin with its signing key, eliminating the possibility of index-based mismatches.

Problem

GetUnspentOutputsForTransaction returned two parallel lists — coins and keys — indexed in lockstep. If the lists ever got out of sync (e.g., by a future refactor that filters one but not the other), the wrong key would sign the wrong input, producing an invalid transaction with no compile-time protection.

Solution

csharp public record SigningCoin(Coin Coin, Key Key);

• Each coin is permanently paired with its derived key at construction time
• Signing loops iterate a single list, accessing sc.Coin and sc.Key
• Error messages now include the outpoint instead of an opaque index
• All test mocks updated to return List<SigningCoin>

Files Changed

File	Change
IWalletOperations.cs	Add SigningCoin record; change return type
WalletOperations.cs	Refactor all signing loops to use SigningCoin
WalletOperationsTest.cs	Update mock setup
FounderTransactionActionTest.cs	Update mock setup
InvestmentIntegrationsTests.cs	Update mock setup
InvestmentOperations.cs (DataBuilder)	Update mock setup
InvestmentOperationsTest.cs	Update mock setup

Testing

All 154 shared tests pass.