refactor: ByteOffset newtype for window/register/ring offsets; named virtio-gpu cmd fields

crates/types/src/address.rs

@@ -68,3 +68,29 @@ impl DataRef {
         }
     }
 }
+
+/// A position in bytes inside a mapped window: a device register, a field
+/// of a ring or command. Adding a byte count moves it along.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+pub struct ByteOffset(usize);
+
+impl ByteOffset {
+    pub const ZERO: Self = Self(0);
+
+    pub const fn new(bytes: usize) -> Self {
+        Self(bytes)
+    }
+
+    pub const fn raw(&self) -> usize {
+        self.0
+    }
+}
+
+impl core::ops::Add<usize> for ByteOffset {
+    type Output = Self;
+
+    /// Saturates: an offset past the end is refused where it is used.
+    fn add(self, bytes: usize) -> Self {
+        Self(self.0.saturating_add(bytes))
+    }
+}