feat: unified gateway route table (static · proxy · remote)

The gateway does two things — reverse-proxy services and serve static
frontends — but the dashboard/API route table only ever showed the proxy
routes, so 'serving a frontend' and 'proxying a service' looked like unrelated
features and castle-app/power-graph-app were invisible in the route view.

Now there's one concept: a route maps an address (path '/foo' or host 'foo.lan')
to a target of one kind — static (a built dist served by file_server), proxy
(a local service port), or remote (a service on another node).

- core: caddyfile.py gains compute_routes() — the single source of truth for
  the route list; generate_caddyfile_from_registry renders it (output byte-for-
  byte identical, verified against the live Caddyfile). Adds a GatewayRoute
  dataclass.
- api: GatewayRoute model → {address, kind, target, name, node}; GET /gateway
  builds from compute_routes (incl. config for static frontends + mesh for
  remote), so the table matches what Caddy actually does.
- app: Gateway panel shows Address · Kind · Target for every route (static
  frontends + host routes now appear); program detail shows 'Reachable at
  /foo/ · served (static)' for static frontends.
- cli: 'castle gateway status' prints the full route table.
- docs: registry.md/design.md/CLAUDE.md describe routes as one concept with
  three target kinds.

core 94 / cli 24 / api 52 green; ruff + app build clean; Caddyfile unchanged.

docs/design.md

@@ -251,8 +251,12 @@ Coordination handles discovery and communication — both between
 programs on a single node and across multiple Castle nodes.
 
 **Intra-node coordination:**
-- Components find each other through the gateway (path-based routing)
-  or direct port access via env vars.
+- Programs find each other through the gateway or direct port access via env
+  vars. A gateway route maps an address (path prefix or host) to a target of one
+  kind: **proxy** (a local service port), **remote** (a service on another
+  node), or **static** (a built frontend's `dist/`, served as files). The same
+  computed route list drives the Caddyfile, `castle gateway status`, and the
+  dashboard, so they always agree.
 - The registry (CLI/API) provides discoverability.
 - No service mesh or message broker required for basic operation.