Isolate execution observations and preserve injected error categories

docs/internal/observation.md

@@ -11,6 +11,12 @@ Prepared execution starts only after its private payload is atomically claimed;
 the facade receives that invocation's recorder privately and emits the terminal
 event after public error mapping. Failed claims and preparation emit nothing.
 
+The facade masks inherited recorder and prepared-claim state when entering
+preparation or execution. Delegated calls from an injected collaborator retain
+the caller's cancellation, deadline, and unrelated context values, but do not
+reuse the enclosing invocation's accounting. Only the new request's observer
+(or the claimed handle's retained observer) enables its observation lifecycle.
+
 A recorder keeps only counters, resolved identity fields, and the selected
 callback. It has no global history, queue, goroutine, or mutex held across a
 callback. The caller supplies normalized failure metadata; the recorder never