swarm-bao: give each hive-cert consumer its own bao identity

Four units read one path each out of the store, and all four logged in
holding `deploy.bao.clientCertFile` — the hive's own leaf. Bao identifies
a principal by the subject of the certificate it presents, so four
readers behind one certificate were ONE principal, and the only grant
expressible was the union of what the four need: read on
`swarm/agents/*`, `swarm/hives/<hive>/*` and `swarm/services/*`. The unit
fetching Grafana's OIDC client secret could fetch every agent credential
in the swarm; the one fetching this hive's matrix token could fetch
Grafana's. Least privilege was not misconfigured here, it was
unrepresentable.

Each now holds a leaf, a cert-auth role and a policy of its own, and each
policy is the single `secret/data/…` path that unit's own script names —
spelled to the leaf, not to a prefix, the way matrix-ctl's already is.
Following the four exemplars in-tree rather than building a mechanism:
`signLeaf` mints the leaves, `swarm-bao.nix` writes the roles from the
bootstrap token, the consumers name their own pair.

Two of the four are written PER HIVE and two are not, which is the shape
of the paths rather than a preference. A matrix appservice token and a
queue credential live under `swarm/hives/<name>/` and every hive runs a
reader for its own, so one role for all of them would have to be granted
`hives/*` — letting one hive read another's, a reach no hive has today.
An OIDC client secret lives under `swarm/services/<client-id>/` and a
swarm registers each exactly once, so one role each is enough. The
per-hive subjects are `<prefix>-<hive>` and swarm.nix reserves every
composed spelling as a hive name, so a hive cannot be named into another
hive's role.

The shared leaf stays: hive-c0re still passes it into its container, the
`bao` CLI wrapper still defaults to it, and the three
`glue-*-bao-identity.nix` files derive the PKI directory from it.

module-eval-bao-grants gains a negative arm per principal — each pins the
three stanzas the hive's leaf carried and the two wildcards a later
widening would reach for, so a policy that grows fails here rather than
in a store. Plus the consuming side: repointing a unit back at the hive's
leaf would evaluate, deploy and log in, and silently restore the union.

A hive that reads a store on another machine now places one leaf per
principal instead of one shared by four. That cost is the point, and
docs/swarm/secrets.md lists the pairs.

docs/getting-started/setup.md

@@ -170,16 +170,19 @@ What else you need depends on
   restart, with the keys `init` printed.
 
 The store serves TLS, and on a hive that deploys it you need do nothing: a
-first-boot unit mints a CA of the store's own plus the two leaves it signs —
-the store's server certificate and this host's client certificate — and points
-`deploy.bao.serverCertFile`, `.serverKeyFile` and `.clientCaFile` at the store's
-half, `.clientCertFile`, `.clientKeyFile` and `.serverCaFile` at the reader's.
+first-boot unit mints a CA of the store's own plus the leaves it signs — the
+store's server certificate, this host's client certificate, and one per service
+principal — and points `deploy.bao.serverCertFile`, `.serverKeyFile` and
+`.clientCaFile` at the store's half, `.clientCertFile`, `.clientKeyFile` and
+`.serverCaFile` at the reader's, and each principal's own pair at its own leaf.
 
 Those are `mkDefault`s, so naming your own paths wins. Do that when your
 certificates come from a real internal CA; the store has no opinion about
 which. A hive that does **not** deploy the store names the reader's three
-itself: the operator issues that leaf out of band, and it's the one credential
-the store can't hand you, being what opens it. ⚠️ Not the gateway's HTTPS certificates and not the hive CA — this is
+itself, plus a pair for every principal it runs — see
+[per-principal identities](../swarm/secrets.md#per-principal-identities) for the
+list. The operator issues those leaves out of band; they're the credentials the
+store can't hand you, being what opens it. ⚠️ Not the gateway's HTTPS certificates and not the hive CA — this is
 **mTLS between services and the store**, a separate trust domain, because a
 store that took its identity from an authority it itself distributes could
 never come up before that authority.