feat(arr): let backends report billed characters

crates/arr-subs/src/translate.rs

@@ -128,6 +128,15 @@ pub trait Backend: fmt::Debug + Send + Sync {
         BATCH_CHAR_BUDGET
     }
 
+    /// Characters this backend has billed so far, cumulative over its life.
+    ///
+    /// Translators charge per character (DESIGN.md §15); the daily budget
+    /// (#197) spends against what was actually billed rather than an
+    /// estimate of it. A backend that cannot say reports zero.
+    fn characters_billed(&self) -> u64 {
+        0
+    }
+
     /// Translate one batch.
     ///
     /// The reply is expected to carry every cue number the batch carried,