swarm: put the matrix registration token where the reader is granted

`swarm-bao-matrix-token` reads `secret/swarm/matrix/registration-token`
and is refused with `Code: 403 — permission denied`, measured on this
host at 16:17:49Z after a successful cert login.

path.rs makes every swarm path `swarm/<kind>/<name>/…` where Kind is a
closed set of four: agents, hives, services, controller. `matrix` sits
where a kind belongs, so policy.rs's read document — which emits exactly
`swarm/agents/*` and `swarm/hives/<hive>/*` — cannot cover it. The
module's own doc predicted this: "a misspelled kind is a 403 at provision
time rather than anything a compiler sees".

Moves the token to `swarm/hives/<hive>/matrix/registration-token`, built
through principal_prefix(Kind::Hive, …) like its per-hive sibling
queue::agent_client_path. The policy is untouched: render already grants
that prefix. mara chose this over widening the namespace.

The nix reader interpolates hyperhiveCfg.hiveName, with the no-fallback
reasoning glue-bao-tls.nix already gives at its own use of it.

path.rs's MOUNT doc justified itself by citing the old literal, which
this commit deletes; rewritten to cite the nix reader instead.

Scope: this makes the read reachable, not the value present. Nothing
writes that path yet, and a 403 says nothing about presence — the two
are separate findings and only the first is fixed here. No migration:
nothing ever wrote the old path and no read ever succeeded.

Gate: cargo fmt 0, cargo test -p swarm-secret-client 0 — 32 passed
against a 29-passed baseline with the change stashed, so the three new
tests are accounted for rather than assumed.

Refs #4308

nix/module-eval.nix

@@ -1148,6 +1148,22 @@ let
         && lib.hasInfix "exit 1" loginBranch
         && lib.hasInfix "exit 0" (lib.last (lib.splitString "bao kv get" u.script));
     }
+    {
+      # The reader's own grant covers `swarm/hives/<this hive>/*` and
+      # `swarm/agents/*`; a path outside those answers 403, not "no such key".
+      # So the hive segment is what makes the read reachable, and a rename that
+      # drops it looks correct and fails identically on every boot.
+      name = "the matrix token path sits inside the prefix the reader is granted";
+      ok =
+        let
+          s = baoWithMatrix.systemd.services.swarm-bao-matrix-token.script;
+        in
+        lib.hasInfix "secret/swarm/hives/" s
+        && lib.hasInfix "/matrix/registration-token" s
+        # The shape it used to have: `matrix` where a principal kind belongs,
+        # which no grant covers.
+        && !(lib.hasInfix "secret/swarm/matrix/" s);
+    }
     {
       # The doctrine three glue files state, as a property a rewrite has to
       # keep: a client is defined by holding a certificate the store accepts,