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/agent-otel.nix

@@ -18,6 +18,8 @@ let
         ;
     })
     agent
+    journaldSeverityOf
+    journaldSeverityOverwritesText
     runGroup
     ;
 
@@ -56,6 +58,39 @@ let
       name = "the agent forwarder reads the persistent journal, not the runtime one";
       ok = ((agentSettings agentOtel).receivers.journald.directory or null) == "/var/log/journal";
     }
+    {
+      # ⚠️ The direction, and the only part of this mapping a reviewer cannot
+      # check by looking: syslog's PRIORITY counts DOWN in urgency (0 = emerg,
+      # 7 = debug) where the OTEL severity counts UP, so a copy wired across
+      # renders debug as critical while every line still arrives and every
+      # field is populated. Asserted at BOTH ENDS of the table — an inverted
+      # mapping still maps every value to something, so "a severity parser is
+      # configured" passes on the exact defect. The swarm tier's receiver gets
+      # the same case over in ./swarm-otel-core.nix; the two import one file,
+      # and these are what say they still do.
+      name = "the agent forwarder maps PRIORITY to severity the right way up";
+      ok =
+        let
+          sev = journaldSeverityOf (agentSettings agentOtel).receivers.journald;
+        in
+        sev "0" == "fatal"
+        && sev "3" == "error"
+        && sev "4" == "warn"
+        && sev "6" == "info"
+        && sev "7" == "debug";
+    }
+    {
+      # The other half of the same operator, and the half that looks already
+      # handled. Without `overwrite_text` the parser sets the severity NUMBER
+      # from the mapping and leaves the severity TEXT as the raw value it
+      # matched — so `severity_text` reaches the log store as the literal "6".
+      # That is a populated field which passes any check asking merely whether
+      # severity is set, and which nothing renders as a level. VictoriaLogs has
+      # no ingest parameter naming a level field, so the text is the whole
+      # interface.
+      name = "the agent forwarder writes a level name, not the raw priority digit";
+      ok = journaldSeverityOverwritesText (agentSettings agentOtel).receivers.journald;
+    }
     {
       # The hop's two ends: what it reads, and where what it reads goes.
       # The endpoint is compared against the value the fixture handed the