Cross-hive trust was O(n²) hand-pinning: every hive had to name every
peer's CA. A swarm root makes it O(1) — trust the root once and every
present and future peer validates.
The root is generated by a new `swarm-ca` unit on a single-host swarm
and operator-provided otherwise; `swarm.ca.autoConfigure` picks between
them and derives its default from `swarm.peers` being empty, so "all on
one host" is read off the deployment rather than remembered. Both modes
produce the same artifacts in the same places, so splitting hosts later
is moving the service dirs, not switching code paths. The root key never
enters the nix store, and the root is never regenerated automatically —
replacing it invalidates every peer at once.
Each hive CA carries `nameConstraints` pinned to that hive's domain, so
a leaked hive CA can only mint names inside its own subdomain, enforced
by verifiers rather than by convention.
`ca.pem` was serving as both the issuer and the anchor consumers trust;
those are the same file only while it is self-signed. openssl will not
terminate a chain at a trusted cert that isn't self-signed (rustls and
Go will), so the promotion would have broken some consumers and not
others. `hive-tls-ca` now also writes `trust-bundle.pem` — the hive CA
plus whatever it is rooted at — and every anchor consumer reads that:
agents, the CI and forge containers, and the peer-config recipe. On a
hive with no swarm root the bundle is just that CA, so nothing consuming
it needs a mode to branch on.
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