MidnightSdkFactory

Seam for constructing a raw MidnightSdk from a WalletConfig + seed.

Exists so MidnightSdkProvider's ownership state machine (build-once, rebuild-on-change, seed-wipe, teardown) is unit-testable without standing up real HD derivation / Room / indexer sockets — tests supply a fake that returns a mock SDK. The production binding lives in WalletRuntimeModule.