swarm-bao: run the swarm's secret store in a container

OpenBao in a swarm-bao nixos-container, unsealed by the host TPM through
the built-in pkcs11 seal, with shamir as the documented opt-out for
hardware that has no TPM. The seal guard asks the package whether it was
built with an HSM rather than reading its version, and falls back to the
version cliff only when a package advertises no tags -- it fails closed.

Every listener serves TLS: loopback unconditionally, because the host
running the store is always one of its readers, plus whatever
extraListenAddresses names, because which network the other hives share
is a deployment fact. Client-certificate verification turns on only when
clientCaFile says what to trust.

The store's identity is an input, and nothing in THIS module fills it
in. Service-to-store mTLS is a separate trust domain from the gateway's
HTTPS certificates and from both CAs in this tree: a store must not take
its identity from an authority it will itself distribute, or it cannot
come up before the thing that issues it. The module asserts when the
paths are unset rather than reaching for whichever CA happens to be
wired; what supplies them on a self-contained deploy is the next commit.

The leaf is copied to the container's state dir by a host unit rather
than bind-mounted: nixos-container refuses to start when a bind source
is missing, and a directory holding a leaf usually holds the CA's key
beside it.

nix/host-modules/default.nix

@@ -24,6 +24,7 @@
     ./hive-tls.nix
     ./otel.nix
     ./swarm-authelia.nix
+    ./swarm-bao.nix
     ./swarm-ca.nix
     ./swarm-nats.nix
     ./swarm-controller.nix