module-eval: the seal, the store's identity, and the reader pairing

The seal choice gates the TPM machinery, in both directions.

The store's three certificate paths are a function of the glue: present
when the store is deployed, and an operator's own path beats the
mkDefault. The absence arm this replaces asserted the store defaults NO
path -- true while nothing supplied one, false the moment something did.

The reader unit is a function of the PAIRING: present with store and
homeserver together, absent with the store alone. That pair is what
makes it glue rather than a feature of either side, and it is the case
that fails if a later change wires the reader to one of them.

nix/module-eval.nix

@@ -71,6 +71,26 @@ let
   bare = hive { };
   withCi = hive { deploy.forgejo.ci.enable = true; };
 
+  baoPkcs11 = hive {
+    deploy.bao.enable = true;
+    deploy.bao.seal = "pkcs11";
+  };
+  baoShamir = hive {
+    deploy.bao.enable = true;
+    deploy.bao.seal = "shamir";
+  };
+  baoExplicitCerts = hive {
+    deploy.bao.enable = true;
+    deploy.bao.serverCertFile = "/etc/pki/bao.pem";
+    deploy.bao.serverKeyFile = "/etc/pki/bao-key.pem";
+  };
+  # The store and a service that reads from it, versus the store alone. The
+  # pair is what makes the reader's absence arm mean anything.
+  baoWithMatrix = hive {
+    deploy.bao.enable = true;
+    deploy.matrix.enable = true;
+  };
+
   # A priority collision is a property of the *option*, not
   # of the merged value's interior — nix throws the moment the value is
   # demanded at all, so `seq`-ing each `serviceConfig` value to WHNF is
@@ -134,6 +154,57 @@ let
       name = "the CI container's unit definitions merge without a priority collision";
       ok = forceCiServiceConfigs;
     }
+    {
+      # The store's seal is spread over five gates — the stanza, the
+      # provisioning unit, a bind mount, a device and an EnvironmentFile.
+      # Rendering only some of them is the dangerous state: a store that
+      # says hardware-backed and seals with a software key, which no
+      # assertion can catch because every value is individually valid.
+      name = "a shamir store renders no TPM provisioning unit";
+      ok = !(baoShamir.systemd.services ? swarm-bao-token);
+    }
+    {
+      # Presence control for the case above. Without it, a typo in the
+      # option name would satisfy the absence arm forever.
+      name = "a pkcs11 store renders the TPM provisioning unit";
+      ok = baoPkcs11.systemd.services ? swarm-bao-token;
+    }
+    {
+      # The store's mTLS identity is a separate trust domain from both CAs in
+      # this tree, because it must not come from an authority the store will
+      # itself distribute. What supplies it is the glue, which mints a CA of
+      # the store's own — so an enabled store has all three paths, and if this
+      # ever reads null again the store stops coming up on its own.
+      name = "a deployed store is given its own certificate, key and client CA";
+      ok =
+        let
+          b = baoPkcs11.services.hyperhive.deploy.bao;
+        in
+        b.serverCertFile != null && b.serverKeyFile != null && b.clientCaFile != null;
+    }
+    {
+      # Everything the glue sets is `mkDefault`, and this is the case that
+      # says so: a deployment whose certificates come from somewhere the glue
+      # has never heard of must win. Also the presence control for the case
+      # above — a renamed option would read `null` on both and satisfy
+      # neither, but only this one names a value.
+      name = "an operator's own certificate path beats the glue's default";
+      ok = baoExplicitCerts.services.hyperhive.deploy.bao.serverCertFile == "/etc/pki/bao.pem";
+    }
+    {
+      # The store's first reader. Its unit belongs to the pairing, not to
+      # either service: matrix must not learn the store exists, and the store
+      # must not know who reads it.
+      name = "a store deployed beside the homeserver fetches its registration token";
+      ok = baoWithMatrix.systemd.services ? swarm-bao-matrix-token;
+    }
+    {
+      # Absence arm. A store with nothing to serve renders no reader, so the
+      # unit is a function of the PAIRING rather than of the store — which is
+      # the property that makes it glue instead of a feature of either side.
+      name = "a store with no homeserver beside it renders no token reader";
+      ok = !(baoPkcs11.systemd.services ? swarm-bao-matrix-token);
+    }
   ];
 
   bad = builtins.filter (c: !c.ok) cases;