iOS: test the locked-keychain path in RootKeyStore

Context

PR #36 added RootKeyStore.swift which throws RootKeyError.interactionNotAllowed when SecItemCopyMatching returns errSecInteractionNotAllowed — the case where the device has been booted but never unlocked since.

The XCTest suite added in PR #36 covers round-trip and wrong-length, but not the locked-keychain path. That branch is reachable in production (cold reboot, app launches via background trigger before user has unlocked once) and has its own native ERROR transition (phase: "rootkey"), so it's worth pinning down with a test.

Why deferred

Simulating errSecInteractionNotAllowed in XCTest is fiddly:

• The simulator's keychain is always "unlocked" once the simulator has booted. There's no public API to lock it.
• One workaround is creating an item with kSecAttrAccessible = kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly on a simulator without a passcode, but the resulting error is errSecAuthFailed, not errSecInteractionNotAllowed.
• The cleanest approach is probably a small protocol-based mock injected at RootKeyStore construction (e.g. a KeychainAccessor typealias), with the production path going through the real SecItem* functions and the test using a fake that returns the desired OSStatus.

That refactor is a couple of hundred lines and changes the public API of RootKeyStore. PR #36 review explicitly deferred it.

Acceptance

• RootKeyStore accepts a keychain-accessor seam (protocol or closure) for tests.
• XCTest covers: load returns nil + locked → throws .interactionNotAllowed.
• XCTest covers: store while locked → throws .interactionNotAllowed.
• Production callers continue to use the real Keychain by default (no behaviour change).

Start with RootKeyStore.swift and the XCTest suite added in PR #36, then review the production SecItem* path and RootKeyStore construction. Add the test seam without changing the default Keychain behavior, and cover locked load and store cases returning .interactionNotAllowed.