feat: kernel W^X (per-section perms, firmware unmapped, stack guard, boot check), halt on kernel faults, region W^X across processes

crates/domain/src/error.rs

@@ -31,6 +31,9 @@ pub enum KernelError {
     /// The target virtual range already holds a mapping.
     AddressInUse,
     InvalidRegion,
+    /// The region is mapped executable somewhere and the request would map
+    /// it writable, or the other way round (W^X across processes).
+    WritableAndExecutable,
     /// The object a capability would refer to no longer exists.
     NoSuchObject,
     /// The capability names the wrong kind of object for this operation.