Reuse validation costs when fitting and batching

docs/internal/validators.md

@@ -142,7 +142,8 @@ payload token estimate. If one enriched item exceeds the configured limit, optio
 neighbors are removed from the end first, then glossary entries from the end,
 until it fits or only required candidate data remains. Required before/after text
 is never truncated. Existing oversized-candidate handling still applies if that
-required data does not fit. Context is supplemental, not a promise that every
+required data does not fit. The built-in estimator reuses encoded fragment
+counts for fitting and batching. Context is supplemental, not a promise that every
 relevant definition or neighboring utterance always fits. This uses the existing
 payload estimator, not a new guarantee about the provider's full context window.