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/lib.nix

@@ -140,6 +140,34 @@ let
 
   agentHarness = machine: machine.systemd.services.hive-agent;
 
+  # Reads a journald receiver's rendered operator list back as a plain
+  # `PRIORITY -> severity-name` function, so a case can ask what a given
+  # priority actually maps to rather than matching on the shape of the config
+  # that produces it. Shared because two tiers run that receiver — the agent
+  # container over its own journal, the swarm collector over the host's — from
+  # one imported mapping, and the case that says so has to reach both.
+  # `null` for a receiver carrying no severity parser at all, which is the
+  # state that mapping replaced.
+  journaldSeverityOf =
+    receiver: priority:
+    let
+      parser = lib.findFirst (o: o.type or null == "severity_parser") null (receiver.operators or [ ]);
+      # The mapping is `severity-name -> value or list of values`; invert it
+      # into `value -> severity-name` so a lookup is by priority.
+      hits = lib.attrNames (lib.filterAttrs (_: v: lib.elem priority (lib.toList v)) parser.mapping);
+    in
+    if parser == null || hits == [ ] then null else lib.head hits;
+
+  # Whether that same parser rewrites the severity TEXT. Its own reader,
+  # because it is a separate way for the mapping to be there and useless: see
+  # the case that asserts it.
+  journaldSeverityOverwritesText =
+    receiver:
+    let
+      parser = lib.findFirst (o: o.type or null == "severity_parser") { } (receiver.operators or [ ]);
+    in
+    parser.overwrite_text or false;
+
   # The enables that switch owns. ⚠️ `otel` is the per-hive collector's own
   # option and is NOT under `deploy` — spelled at the wrong path it would be
   # undeclared rather than false, and a roster that quietly loses a member is
@@ -197,6 +225,8 @@ in
   inherit baoSettings;
   inherit otelSettings;
   inherit agentHarness;
+  inherit journaldSeverityOf;
+  inherit journaldSeverityOverwritesText;
   inherit swarmServiceEnables;
   inherit runGroup;
 }