Use local builders for rendered templates

docs/internal/sources.md

@@ -143,7 +143,9 @@ between artifact conversion chunks, around each message, and before publishing
 the complete prompt. It cannot interrupt template work already in progress and
 never publishes a partial prompt after observing cancellation. It validates and
 canonicalizes message roles before carrying roles, session IDs, and cache
-control into the rendered prompt. The
+control into the rendered prompt. Message and session templates execute into
+per-call string builders, so their rendered strings do not require a final
+buffer-to-string copy. The
 [renderer tests](../../internal/prompt/renderer_test.go) own rendering behavior.
 
 ## Schemas And Output Validation