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/checks.nix

@@ -62,6 +62,14 @@ in
     version = "0.1.0";
     cargoTestExtraArgs = "--workspace";
     HIVE_ASSETS_DIR = "${self.packages.${system}.assets}/share/hyperhive";
+    # The reserved-name blacklist moved out of the Rust tree and into
+    # `reserved-names.nix`, so the test that pins the message layer's
+    # sentinels against it can only run if nix hands it the same list the
+    # daemons get. Exported here and in `devshell.nix`, and the test PANICS
+    # rather than skipping when the variable is missing: a drift test that
+    # quietly does nothing is worse than no drift test, because it still
+    # shows up green.
+    HIVE_RESERVED_NAMES = pkgs.lib.concatStringsSep " " (import ./reserved-names.nix);
   };
 
   # Rustdoc gate. Builds the workspace's docs and turns rustdoc's own