Add spa flag for static deployments; fix Hugo navigation

Static (manager: caddy) deployments always emitted the SPA fallback
`try_files {path} /index.html`, which serves the root index.html for
every unmatched path. That's correct for React/Vite apps but swallows a
multi-page content site's in-page links back to the homepage, so a Hugo
site (payne.io) couldn't navigate off its landing page.

Add an explicit `spa: bool` on CaddyDeployment (default True, preserving
existing SPA behavior). When False, the gateway serves plain file_server
— resolving directory indexes (/posts/ -> /posts/index.html) and 404ing
missing paths. Threaded through the registry snapshot + mesh and the
route computation so both the internal wildcard and public apex blocks
honor it. `castle program create --stack hugo` now sets spa: false.

core/src/castle_core/registry.py

@@ -92,6 +92,9 @@ class Deployment:
     # For `static` runner services: the absolute dir the gateway file_servers.
     # Set → the route is `static` (file_server) rather than `proxy` (reverse_proxy).
     static_root: str | None = None
+    # For `static` routes: SPA fallback (True, default) vs content-site serving
+    # (False, Hugo/multi-page). See CaddyDeployment.spa.
+    spa: bool = True
     base_url: str | None = None
     schedule: str | None = None
     managed: bool = False
@@ -192,6 +195,7 @@ def load_registry(path: Path | None = None) -> NodeRegistry:
             public_host=comp_data.get("public_host"),
             tcp_port=comp_data.get("tcp_port"),
             static_root=comp_data.get("static_root"),
+            spa=comp_data.get("spa", True),
             base_url=comp_data.get("base_url"),
             schedule=comp_data.get("schedule"),
             managed=comp_data.get("managed", False),
@@ -273,6 +277,10 @@ def save_registry(registry: NodeRegistry, path: Path | None = None) -> None:
             entry["tcp_port"] = comp.tcp_port
         if comp.static_root:
             entry["static_root"] = comp.static_root
+        # Only emit when non-default (content-site) — default-True omission keeps
+        # existing registries byte-identical and matches the load-side default.
+        if not comp.spa:
+            entry["spa"] = comp.spa
         if comp.base_url:
             entry["base_url"] = comp.base_url
         if comp.schedule: