harden: enforceGatedOwnerKeyForValue is now private(set)

The go-live gated-key value-signing wall (signForValue refuses a non-biometric-gated
owner key when this flag is on) was a plain settable static. Nothing in the app or
tests assigns it at runtime, so making it private(set) changes no behavior today — it
just removes the ability for any code (including the Morpheus advisor) to disarm the
wall by writing it false. Go-live still arms it by flipping the default. Pure-win
hardening, independent of Morpheus.

Author: Neo

Core/Wallet/SecureEnclaveManager.swift

@@ -23,7 +23,7 @@ final class SecureEnclaveManager {
     /// tests — which use ungated keys — stay green). Flip to `true` at go-live so the weak path is
     /// REFUSED and a legacy ungated owner key can never move funds. Identity-proof signing is
     /// unaffected — it goes through `sign()`, not `signForValue()`.
-    static var enforceGatedOwnerKeyForValue = false
+    static private(set) var enforceGatedOwnerKeyForValue = false
 
     enum KeyError: Error, LocalizedError {
         case keychainStatus(OSStatus)