fix: restore V3 HTTP observability

Project one typed HttpExecutionObservation per logical V3 execution through a
closed composition-root projector: only registered diagnostic context keys and
bucketed values reach the sinks, and terminal non-abort failures now emit
exactly one api.request.failed telemetry event. Caller cancellation and scope
fencing record a diagnostic but never a failure event.

routeId becomes a required input at the installed operation-executor boundary
so the feature gateway's low-cardinality route identity survives to the sink.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/features/reference-feature/adapters/create-reference-feature-input.ts

@@ -23,7 +23,8 @@ export type InstalledContractOperationExecutor = Readonly<{
   execute(
     operationId: string,
     input: unknown,
-    context?: Readonly<{
+    context: Readonly<{
+      routeId: string;
       signal?: AbortSignal;
       intent?: MutationIntent;
     }>,
@@ -48,6 +49,9 @@ export function createReferenceFeatureInstalledInput(context: Readonly<{
         operationId,
         input,
         {
+          // §7.4. The gateway owns the low-cardinality route identity; losing
+          // it here is what made every V3 diagnostic unattributable.
+          routeId: request.routeId,
           ...(signal === undefined ? {} : { signal }),
           ...(intent === undefined ? {} : { intent }),
         },