swarm-bao: make the reader's identity declarable, not just the store's

`swarm-bao.nix` declared the store's half of the mTLS pair as options —
`serverCertFile`, `serverKeyFile`, `clientCaFile` — and left the reader's
half as a literal inside `glue-bao-tls.nix`, which only runs where
`deploy.bao.enable` is set. A hive that did not host the store therefore
could not read from it and could not be pointed at a certificate even
when one had been placed by hand.

Adds `clientCertFile`, `clientKeyFile` and `serverCaFile` beside their
three server siblings, `mkDefault`ed by the glue to the leaf it already
mints, and moves `glue-matrix-bao-token.nix` onto them. Its gate becomes
"this host holds an identity" rather than "the store is a neighbour",
and the unit ordering that names store-local units is now conditional --
`Requires=` on an absent unit fails the job.

`serverCaFile` is separate from `clientCaFile` on purpose: one is the
store choosing which readers to trust, the other a reader choosing which
store to trust. Self-signing collapses them to one file, which is a
property of that deployment and not of the pairing.

Closes #3855.

docs/swarm/secrets.md

@@ -156,6 +156,10 @@ mints a CA that signs exactly two things, the store's server certificate and a
 reader's client certificate, and distributes nothing. A deployment with a real
 internal CA deletes that file and names its own paths in
 `deploy.bao.serverCertFile` / `clientCaFile`; the store itself has no opinion.
+A hive that reads from a store on **another** machine names the reader's half —
+`clientCertFile`, `clientKeyFile`, `serverCaFile` — and places that leaf by hand.
+It is the one credential that cannot come out of the store, being what opens it;
+everything else a hive needs does.
 
 ## The constraint that decides where the root lives