upstream: a diagnostics episode follows health, and a peer fault carries its cause

src/app.zig

@@ -557,6 +557,13 @@ fn serve(r: cli.Runner, args: cli.RunArgs) !u8 {
 
     var upstreams: upstream_owner.Owner = .init(upstream_generation);
     defer upstreams.deinit(io);
+    upstreams.diagnostics = event_store;
+    // The store outlived the process that wrote it, so every `upstream.exchange`
+    // episode in it describes a pool that no longer exists. This closes the ones
+    // no enabled upstream of this boot can justify, and projects the health of
+    // the ones that remain — a fresh pool is all clear, so a warning that
+    // survives this boot is one this process opened.
+    upstreams.reconcileDiagnostics(io, upstream_generation);
 
     // -----------------------------------------------------------------------
     // per-query state