Detach prepared execution ownership from runners

docs/roadmap/implementation.md

@@ -266,6 +266,10 @@ same-engine views, one-shot/concurrent claim/discard and details after use.
 Session Unicode/trim/content/wire/hash tests stay unchanged in meaning. Reference
 analysis, not a GC-timing assertion, establishes that the token retains no runner.
 
+Completion: prepared handles retain only a private nonzero-sized runner identity
+token, including after use or discard; accepted session strings own their
+storage. Focused prepared/session race tests pass on Go 1.27.3.
+
 ## Stage 10: Declare Validation Preparation As The Internal Dependency
 
 **Catalog:** 21, interface part. **Files:** validator constructors, engine Options,