types: let nix own the reserved-name blacklist

One list, in nix/reserved-names.nix, handed to everything that needs it
as HIVE_RESERVED_NAMES. Keeping it current becomes a config change
rather than a rebuild, and hive names and agent names -- one namespace
going forward -- are checked against the same file: swarm-otel.nix's
hand-written reservedOwners is gone.

Whitespace-separated rather than JSON, deliberately, unlike the
structured env vars beside it. Every entry is an Ident ([a-z0-9-]), so
whitespace cannot occur inside a name and the encoding is provably
lossless; JSON would mean either a parser dependency in a crate whose
purpose is to have none, or a copy of the parse in every consumer.

An UNSET variable is not "nothing is reserved". Both creation sites log
an error and return a warning saying the check did not run, so a
misconfigured deployment says so instead of silently accepting every
name. A blank value folds into unset: nix always renders a non-empty
list, so present-but-empty is a rendering fault, not a declaration.

Two guards whose subject moved out of their own file now assert their
own case is still in it, because a guard that can be retired by an edit
elsewhere is not a guard:

- swarm-otel.nix asserts reserved-names.nix still contains its
  swarmTierName.
- hive-sh4re's sentinel drift test PANICS when the variable is missing
  rather than skipping -- a drift test that quietly does nothing still
  reports green. checks.nix and devshell.nix both export it so CI and a
  local cargo test agree. Verified as a pair: with the variable set, 8
  tests pass; with it unset, exactly the 4 drift tests fail and the
  unrelated ones still pass.

nix/host-modules/hive-c0re/environment.nix

@@ -143,6 +143,21 @@ in
   # mandatory, so this is unconditional (the whole env block is already
   # gated on hyperhive being enabled). See `docs/gateway.md::HIVE_FORGE_URL`.
   HIVE_FORGE_URL = "http://${config.services.hyperhive.swarm.forge.domain}";
+
+  # The one blacklist of names an agent may not take — see
+  # `nix/reserved-names.nix`, which is also read by the swarm controller, by
+  # the swarm collector's owner assertion, and by the test suite. Nix owns it
+  # so that keeping it current is a config change, not a rebuild of a binary.
+  #
+  # Whitespace-separated rather than JSON: every entry is an `Ident`
+  # (`[a-z0-9-]`), so a space can never occur inside a name and the encoding
+  # cannot be lossy. Spelling it as JSON would put a parser in the crate whose
+  # whole point is to have no dependencies.
+  #
+  # Unconditional on purpose. The consumer treats an ABSENT variable as "I was
+  # never told" and says so out loud, which is the correct reading — but it is
+  # a reading no correctly-built hive should ever have to make.
+  HIVE_RESERVED_NAMES = lib.concatStringsSep " " (import ../../reserved-names.nix);
 }
 //
   lib.optionalAttrs

nix/host-modules/swarm-controller.nix

@@ -648,6 +648,13 @@ in
         # `swarm.peerHives`, `swarm.hives` minus this hive) rather than
         # peers-minus-self. Consumed by `GET /api/hives`
         # (swarm-controller/src/main.rs::load_hives).
+        # The one blacklist of names an agent may not take, shared verbatim
+        # with hive-c0re and with the collector's owner assertion — see
+        # `nix/reserved-names.nix`. Whitespace-separated rather than JSON
+        # like its neighbour below, because every entry is an `Ident` and so
+        # cannot contain a space; the neighbour carries objects and has no
+        # such option.
+        HIVE_RESERVED_NAMES = lib.concatStringsSep " " (import ../reserved-names.nix);
         SWARM_CONTROLLER_HIVES = builtins.toJSON (
           lib.mapAttrsToList (name: h: {
             inherit name;

nix/host-modules/swarm-otel.nix

@@ -55,9 +55,16 @@ let
   # repeated at each site would let the guard and the config drift apart, which
   # is the failure this guard exists to prevent.
   swarmTierName = "swarm";
-  # Every `<owner>` this module claims for itself. One entry today; a second
-  # swarm-tier pipeline would be added here and inherit the check for free.
-  reservedOwners = [ swarmTierName ];
+  # Every `<owner>` no hive may take. Read from `nix/reserved-names.nix`, the
+  # same file the daemons are handed as `HIVE_RESERVED_NAMES`, because agent
+  # names and hive names are ONE namespace going forward — a locally-owned
+  # list here would be a second copy to keep in step, which is the failure a
+  # single blacklist exists to prevent.
+  #
+  # The assertion below still checks the string this module emits: the file
+  # is asserted to CONTAIN `swarmTierName`, so a rename that dropped it from
+  # the file would be an eval error rather than a silently missing guard.
+  reservedOwners = import ../reserved-names.nix;
 
   # A published target is declared as ONE url, because that url is also the
   # audience its token is minted for — but prometheus wants the same fact in
@@ -581,6 +588,23 @@ in
           List the swarm's hives.
         '';
       }
+      {
+        # The blacklist now lives in a shared file, so this module no longer
+        # controls its contents — and a guard whose subject can be edited
+        # elsewhere has to assert that its own case is still in there. Without
+        # this, deleting one line from `reserved-names.nix` would silently
+        # retire the check below rather than fail anything.
+        assertion = lib.elem swarmTierName reservedOwners;
+        message = ''
+          nix/reserved-names.nix no longer contains '${swarmTierName}', which
+          the swarm collector needs reserved: it names components
+          `<kind>/<owner>` and uses the hive name as the owner, so a hive
+          called '${swarmTierName}' would replace the swarm tier's own
+          pipelines and lose its own.
+
+          Put it back, or give this module a different swarmTierName.
+        '';
+      }
       {
         # A hive whose name is one this module claims for itself collides in
         # the collector's component namespace, and `//` resolves it silently: