Project terminal execution observations into shared reports

docs/internal/adapters.md

@@ -23,6 +23,8 @@ described by its tagged
   and errors to HTTP DTOs.
 - `internal/format` renders `promptkit.PreparedRun` values as deterministic text
   or JSON.
+- `internal/report` maps Promptkit's terminal execution event to a small
+  application-owned value without invoking the engine or writing output.
 
 ## Wiring Flows
 
@@ -71,6 +73,11 @@ Failure mapping uses `errors.Is` against Promptkit's public sentinels and the
 HTTP reader's Scriptorium-owned containment and size errors. Wrapped reader
 errors preserve their identity through Promptkit's artifact-load boundary.
 
+The report collector is per invocation. It ignores nonterminal events and
+copies terminal aggregates once, including usage presence and completeness.
+It retains no history or shared state; adapters decide whether to attach it and
+how to present its snapshot.
+
 ## Package-Local Guarantees
 
 - Adapters contain no copied framework types or orchestration.