nix(tls): host hive-CA + gateway leaf for self-signed mode

Replace the gateway's bare in-container self-signed leaf with a leaf
signed by a host-held hive CA. A bare self-signed leaf is its own trust
anchor, so every regeneration is a fresh anchor every consumer would have
to re-trust, and a runtime-generated in-container leaf cannot be wired
into an agent's build-time trust store at all. A stable CA fixes both: a
single anchor that agents and federation peers trust once, surviving leaf
rotation.

New hive-tls module: a host oneshot generates a long-lived CA (default
~20y) under services.hyperhive.tls.stateDir and signs a gateway leaf
(default ~10y, SAN covering the bare domain, forge., matrix. and the
wildcard). It is ordered before the gateway container so the leaf exists
when nginx starts. Active only when the gateway uses self-signed TLS
(default) and a domain is set; inert under operator-cert or ACME modes.

Gateway: bind-mount the host CA dir read-only at /run/hive-ca; the
existing in-container cert unit now imports the host leaf into nginx's
state dir (copy as root, key left root:nginx 0640 for the pre-start
config test) instead of generating one. Cert/key paths nginx serves are
unchanged.

Foundational step toward agent + federation trust of self-signed hives;
no behaviour change for agents yet (they still reach the forge over plain
http on port 80). Eval-proven across self-signed, certDir and the inert
default paths.

nix/modules/hive-c0re.nix

@@ -120,6 +120,7 @@ in
     ./hive-gateway.nix
     ./hive-matrix.nix
     ./hive-network.nix
+    ./hive-tls.nix
   ];
 
   # Top-level hyperhive enable flag. When true, automatically enables