otel.nix: trim the StartLimit comment block to the load-bearing points

Cut ~27 lines of blackout-measurement and cross-reference narrative
(already in the PR body / issue) down to the three things a reader
actually needs at this call site: the [Unit]-vs-[Service] trap, why
Restart is absent, and the window-vs-burst constraint.

nix/host-modules/otel.nix

@@ -626,21 +626,35 @@ in
         };
       };
 
-      # `LoadCredential` and not a copy-oneshot: this collector is a HOST
-      # unit, so there is no container boundary to cross and therefore no
-      # reason for a second on-disk copy of the secret. systemd hands it to
-      # the process in a private tmpfs and exports the directory, which is
-      # what the config above names.
-      systemd.services.opentelemetry-collector = lib.optionalAttrs senderAuth {
-        serviceConfig.LoadCredential = [ "${credName}:${otel.clientSecretFile}" ];
+      systemd.services.opentelemetry-collector = lib.mkMerge [
+        # ⚠️ `StartLimit*` are `[Unit]` settings; systemd IGNORES them under
+        # `[Service]` — in `serviceConfig` they render, deploy and do
+        # nothing. `checks.module-eval-hive-otel` pins them in `unitConfig`.
+        # `Restart` is deliberately NOT set: nixpkgs' own module defines it
+        # at normal priority, so a second definition is a module-system
+        # conflict. The window must exceed `RestartSec × burst`.
+        {
+          startLimitBurst = 12;
+          startLimitIntervalSec = 120;
+          serviceConfig.RestartSec = 5;
+        }
 
-        # `requires`, not merely `after`: ordering without gating leaves the
-        # collector starting anyway and failing on its own `LoadCredential`,
-        # which is the failure the wait exists to prevent. Same reasoning
-        # `lib/hive-ca-trust.nix` records for its bundle consumers.
-        requires = [ "${secretWaitUnit}.service" ];
-        after = [ "${secretWaitUnit}.service" ];
-      };
+        # `LoadCredential` and not a copy-oneshot: this collector is a HOST
+        # unit, so there is no container boundary to cross and therefore no
+        # reason for a second on-disk copy of the secret. systemd hands it to
+        # the process in a private tmpfs and exports the directory, which is
+        # what the config above names.
+        (lib.mkIf senderAuth {
+          serviceConfig.LoadCredential = [ "${credName}:${otel.clientSecretFile}" ];
+
+          # `requires`, not merely `after`: ordering without gating leaves the
+          # collector starting anyway and failing on its own `LoadCredential`,
+          # which is the failure the wait exists to prevent. Same reasoning
+          # `lib/hive-ca-trust.nix` records for its bundle consumers.
+          requires = [ "${secretWaitUnit}.service" ];
+          after = [ "${secretWaitUnit}.service" ];
+        })
+      ];
 
       # Wait for the credential rather than racing it.
       #

nix/module-eval/hive-otel.nix

@@ -111,6 +111,27 @@ let
         in
         !(m ? address) && (lib.head m.readers).pull.exporter.prometheus.port == 8888;
     }
+    {
+      # A bind that loses the race with the outgoing process is the whole
+      # failure: nixpkgs ships `Restart = "always"` with no `RestartSec`, so
+      # without this the unit spends its five default attempts inside two
+      # seconds and lands in `start-limit-hit`, where it stops retrying. The
+      # third clause is the one that has to hold — `StartLimit*` are `[Unit]`
+      # settings that systemd ignores under `[Service]`, so a bound written
+      # into `serviceConfig` renders, deploys and does nothing. Asserted
+      # where nixpkgs puts it rather than where it was written, same as
+      # ./bao-grants.nix.
+      name = "the hive collector backs off a failed bind from [Unit], not [Service]";
+      ok =
+        let
+          u = hiveOtel.systemd.services.opentelemetry-collector;
+        in
+        u.serviceConfig.RestartSec or 0 > 0
+        && toString u.unitConfig.StartLimitBurst == "12"
+        && !(u.serviceConfig ? StartLimitBurst)
+        # The window has to outlast every attempt, or the burst is unreachable.
+        && u.startLimitIntervalSec or 0 > u.serviceConfig.RestartSec * u.startLimitBurst;
+    }
     {
       # Both collectors share a network namespace whenever they are
       # co-located, and this port appears in no config the port-collision