fix(swarm): say certFingerprint was removed, not that it does not exist

A deployed config still set `swarm.hives.<hive>.certFingerprint`, deleted
along with the dashboard feature it served. The module system's answer was
`The option ... does not exist`, which tells an operator nothing about why
it went, whether it moved, or what replaces it.

Re-declared invisible and internal, with a top-level assertion naming the
hives that still carry it and explaining that the swarm root CA replaces
per-hive leaf pinning.

`lib.mkRemovedOptionModule` cannot do this job, and it is worth writing
down why: neither of its halves survives the move into a submodule. Its
`apply = throw` fires only when the value is read, and nothing reads this
any more — that being the point of removing it. Its `config.assertions`
half would land on a submodule that declares no `assertions` option. It is
a top-level tool. This is the same shape swarm-peers-removed.nix already
uses for the analogous `peers.<hive>.caCert`.

An error rather than a warning, because re-declaring the option is what
stops the unhelpful message — and on its own that would turn a config that
used to fail into one that quietly evaluates with the setting ignored,
which is worse than the error it replaced.

nix/host-modules/swarm.nix

@@ -46,6 +46,13 @@ let
   # implicitly. Left out, its vhost falls back to the hive leaf and the
   # swarm's front page opens with a name mismatch.
   ++ lib.optional swarmCfg.ui.enable swarmCfg.ui.domain;
+
+  # Hives whose entry still carries the removed `certFingerprint`. Scanned
+  # here, at top level, because that is the only place an assertion about a
+  # submodule field can actually live — see the option's own comment.
+  pinnedHives = lib.attrNames (
+    lib.filterAttrs (_: hive: hive.certFingerprint != null) swarmCfg.hives
+  );
 in
 {
   options.services.hyperhive.swarm.hives = lib.mkOption {
@@ -128,6 +135,33 @@ in
                 silently excluded from `wg-hive`).
               '';
             };
+
+            # Removed, and re-declared only so an existing definition
+            # produces an error that says *what happened*. Without it the
+            # module system says `The option ... does not exist`, which
+            # tells an operator nothing about why it went or what replaced
+            # it — and this field was set on real deployments.
+            #
+            # ⚠️ `lib.mkRemovedOptionModule` cannot do this job here, and
+            # neither of its two halves survives the move into a submodule:
+            # its `apply = throw` fires only when the value is READ, and
+            # nothing reads this any more (that being the point of removing
+            # it); its `config.assertions` half would land on a submodule
+            # that declares no `assertions` option at all. It is a
+            # top-level tool. The assertion below is the working shape, and
+            # it is the same one ./swarm-peers-removed.nix uses for the
+            # analogous `peers.<hive>.caCert`.
+            certFingerprint = lib.mkOption {
+              type = lib.types.nullOr lib.types.str;
+              default = null;
+              visible = false;
+              internal = true;
+              description = ''
+                Removed. Trust inside a swarm comes from the swarm root CA
+                (`services.hyperhive.swarm.ca`, docs/swarm/ca.md), which
+                replaces per-hive leaf pinning entirely.
+              '';
+            };
           };
         }
       )
@@ -232,6 +266,38 @@ in
           Declared hives: ${lib.concatStringsSep ", " (lib.attrNames swarmCfg.hives)}
         '';
       }
+      {
+        # An error rather than a warning, deliberately. Re-declaring the
+        # removed option above is what stops "option does not exist"; on
+        # its own it would also turn a config that used to FAIL into one
+        # that quietly evaluates with the setting ignored, which is a
+        # worse answer than the unhelpful error it replaces. The operator
+        # asked for a removal error, so this stays fatal until the field
+        # is gone from their config.
+        #
+        # Wording follows `lib.mkRemovedOptionModule`'s, so it reads like
+        # every other removal the module system reports.
+        assertion = pinnedHives == [ ];
+        message = ''
+          The option definition `services.hyperhive.swarm.hives.<hive>.certFingerprint'
+          no longer has any effect; please remove it.
+
+          Still set on: ${lib.concatStringsSep ", " pinnedHives}
+
+          It pinned a peer hive's TLS leaf for hive-c0re's own peer HTTPS
+          checks, and was removed along with the dashboard feature it
+          existed to serve — nothing else ever consumed it.
+
+          There is no replacement, and none is needed for a hive inside
+          this swarm: trust comes from the swarm root CA
+          (services.hyperhive.swarm.ca — see docs/swarm/ca.md), which
+          every hive chains to, so one anchor replaces per-hive pinning.
+          What that genuinely drops is trusting a hive whose root this
+          swarm does NOT own — another swarm's, or one keeping its own
+          CA. That is a cross-swarm problem and wants a mechanism
+          designed for it.
+        '';
+      }
     ];
   };