swarm CA: retire the host-held root CA, the store holds the only anchor

The secret store's pki root issues every hive's gateway leaf and the
swarm's service names, so the root swarm-ca.service generated on the
host anchors nothing and its key is a secret outside the store.

- swarm-ca.nix is gone: swarm-ca.service, root.pem and root-key.pem,
  and the swarm.ca.{autoConfigure,stateDir,validityDays} options. A
  definition of any of them fails evaluation with a removal message
  (swarm-ca-removed.nix), and a tmpfiles rule deletes /var/lib/swarm-ca
  on every host. singleHostSwarm no longer sets a swarm.* option.
- hive-tls-ca drops the $root slot: trust-bundle.pem holds the store's
  pki root and a hive CA's public ca.pem/ca-previous.pem only.
- hivectl peer-config no longer prints the scp line for the swarm root;
  the store's pki root is the one step.
- The gateway's cert import no longer stands a hive leaf or a throwaway
  in for a missing swarm-services leaf where it Requires= the issuing
  unit: it fails, naming swarm-services-cert.service. The stand-in stays
  where the gateway does not wait (no swarm name, or the store's UI
  only, whose passthrough must start with the store sealed).

Comments and docs that named swarm-ca.nix, swarm.ca.* or the host root
now name the store's pki root.

Refs #4588

docs/networking/gateway.md

@@ -76,11 +76,11 @@ On by default, listening on `httpsPort` (default 443) beside the plain-http `por
 
 The issuer is the **secret store's `pki` mount**, not a bare self-signed leaf. `hive-tls-resign.service` (from `hive-tls.nix`) asks the store for a gateway **leaf** through this hive's own role, presenting the hive's own store identity (`services.hyperhive.deploy.hive-controller.tls.baoHiveIdentityCertFile` / `baoHiveIdentityKeyFile`), and writes it under `…tls.stateDir` (default `/var/lib/hive-tls`). `hive-gateway-self-signed-cert` then imports the leaf into nginx's state dir (`/var/lib/hive-gateway/tls/{cert,key}.pem`). On a host that runs no hive the import unit installs the swarm-services leaf there instead, or a throwaway self-signed pair until that leaf lands. [`swarm/ca.md`](../swarm/ca.md) has the role, the identity and what a hive without one does.
 
-⚠️ **Keep the import unit.** It does two jobs nginx needs: it re-modes the key to `0640 root:nginx` (nginx's pre-start `nginx -t` runs as the nginx user and fails on the source's `0600 root:root` key), and it makes sure **every cert path the config names exists** — when the swarm-services leaf is missing it installs the hive leaf in its place, or on a host that runs no hive a throwaway self-signed pair. nginx refuses a config naming a missing cert file, so without that fallback one missing leaf takes down every vhost, not just one.
+⚠️ **Keep the import unit.** It does two jobs nginx needs: it re-modes the key to `0640 root:nginx` (nginx's pre-start `nginx -t` runs as the nginx user and fails on the source's `0600 root:root` key), and it puts a file at **every cert path the config names**, because nginx refuses a config naming a missing cert file and one missing leaf would take down every vhost, not just one. Where the gateway waits for the swarm-services leaf, a missing one fails the import unit, naming `swarm-services-cert.service`, and nginx stays down until that unit issues it. Where it doesn't — a host fronting no swarm name, or only the store's UI, which starts its passthrough with the store sealed — the hive leaf stands in until the services leaf lands, or on a host that runs no hive a throwaway self-signed pair.
 
 **Why an issuer, not a bare leaf**: a bare self-signed leaf is its own trust anchor, so every regeneration is a new anchor every consumer must re-trust, and an agent's build sets its trust store once, before any leaf exists. Agents and federation peers trust the store's root once; leaf rotation never breaks them.
 
-**What consumers trust**: `trust-bundle.pem` in the same state dir. It carries the store's `pki` root, plus a hive CA's public `ca.pem` and the swarm root where they exist ([`swarm/ca.md`](../swarm/ca.md#the-trust-bundle)). nginx serves the leaf alone, since its issuer is a self-signed root. Agents (via `security.pki.certificateFiles`), the CI and forge containers and federating peers all read the bundle.
+**What consumers trust**: `trust-bundle.pem` in the same state dir. It carries the store's `pki` root, plus a hive CA's public `ca.pem` where one exists ([`swarm/ca.md`](../swarm/ca.md#the-trust-bundle)). nginx serves the leaf alone, since its issuer is a self-signed root. Agents (via `security.pki.certificateFiles`), the CI and forge containers and federating peers all read the bundle.
 
 **Why on by default**: matrix-dart-sdk (FluffyChat's SDK) fetches `https://<host>/.well-known/matrix/client` and never falls back to plain http, so without TLS the browser client can't bootstrap.