module-eval: pin the severity mapping's direction and the panel

The direction is the part a reviewer cannot check by looking, so it is
asserted at both ends of the table and in both tiers' groups: an inverted
mapping still maps every value to something, and a case that only asks
whether a severity parser exists passes on the exact defect. The reader
that turns a rendered operator list back into a PRIORITY -> name function
lives in lib.nix, since both tiers need it.

The panel is asserted on its query rather than its title, because a panel
that keeps the title and loses the expression renders an empty graph that
looks exactly like zero prioless lines.

nix/module-eval/swarm-otel-core.nix

@@ -18,6 +18,8 @@ let
         ;
     })
     hive
+    journaldSeverityOf
+    journaldSeverityOverwritesText
     runGroup
     otelSettings
     ;
@@ -155,6 +157,34 @@ let
         in
         named != [ ] && lib.all (a: builtins.elem a s.service.extensions) named;
     }
+    {
+      # The host-journal half of the mapping the agent tier's receiver carries
+      # (./agent-otel.nix holds the sibling case and the full reasoning). Both
+      # import one file, so this is what says the swarm collector still does —
+      # the receivers are otherwise unrelated config, and a drifted copy is
+      # silent: every line still arrives, labelled as the wrong thing.
+      #
+      # Asserted at both ends of the table because the direction is inverted
+      # and an inverted copy still maps every value to something.
+      name = "the swarm collector maps the host journal's PRIORITY the right way up";
+      ok =
+        let
+          sev = journaldSeverityOf (otelSettings otelNoStores).receivers.journald;
+        in
+        sev "0" == "fatal"
+        && sev "3" == "error"
+        && sev "4" == "warn"
+        && sev "6" == "info"
+        && sev "7" == "debug";
+    }
+    {
+      # Same reasoning as the agent tier's copy: without this the parser
+      # populates `severity_text` with the raw digit it matched, which passes
+      # any check asking whether severity is set and is not a level anything
+      # renders.
+      name = "the swarm collector writes a level name, not the raw priority digit";
+      ok = journaldSeverityOverwritesText (otelSettings otelNoStores).receivers.journald;
+    }
   ];
 in
 runGroup "swarm-otel-core" cases