module-eval: evaluate shared fixture configs once

Thirty-three module-eval fixtures were a second (or later) full
`nixosSystem` evaluation of a config another fixture already evaluated:
16 shapes defined identically in two to seven cluster files, plus
duplicates inside agent-queue-bao, core-toggle and
swarm-services-switch. Each cluster imports ./lib.nix itself, so nothing
was shared between them.

nix/module-eval/shared.nix now defines each of those 16 shapes once.
nix/checks.nix instantiates it a single time and passes it as `shared`
to the 23 clusters that read one, and each cluster rebinds the fixture
under its existing local name. Assertions are unchanged. Case text
changes only in core-toggle, where two cases built the forge-host
fixture inline and now read the file's `forgeHere`, and in two inline
`let` bindings (bao-otel-collector, grafana) that now take the shared
fixture.

Six folds are value-equal rather than textually equal: they drop an
explicit `deploy.hive-controller.enable = false` or
`deploy.swarm-otel.enable = false` that equals what the option already
resolves to for that config. Nothing under nix/host-modules sets either
option apart from the `mkDefault allSwarmServices` on swarm-otel, and
the only option-priority read there is `domain.highestPrio`.

Part of #5171.

nix/module-eval/matrix-core.nix

@@ -6,6 +6,7 @@
   lib,
   self,
   nixosSystem,
+  shared,
 }:
 let
   inherit
@@ -18,7 +19,6 @@ let
         ;
     })
     hive
-    hiveWithController
     runGroup
     ;
 
@@ -31,11 +31,11 @@ let
 
   # The same hive with the identity taken away, which separates "a homeserver
   # is deployed" from "this host can authenticate to the store".
-  matrixNoBaoIdentity = hiveWithController { deploy.matrix.enable = true; };
+  inherit (shared) matrixNoBaoIdentity;
 
   # A hive host that does not run the homeserver: its hive-c0re reaches the
   # homeserver across the network.
-  matrixElsewhere = hiveWithController { };
+  matrixElsewhere = shared.bareWithController;
   cases = [
     {
       # Absence arm, and what makes the one above able to fail: with no leaf