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.

cli/src/castle_cli/commands/create.py

@@ -41,6 +41,11 @@ STACK_BUILD_OUTPUTS: dict[str, str] = {
     "hugo": "public",
 }
 
+# Stacks whose static output is a multi-page content site (not a single-page app):
+# the gateway resolves directory indexes and 404s missing paths instead of falling
+# back to the root index.html. See CaddyDeployment.spa.
+CONTENT_SITE_STACKS: set[str] = {"hugo"}
+
 # Substrate a stack's apps depend on — seeded as a deployment `requires` at creation
 # so the relationship graph shows it. This keeps `stack` uncoupled from the runtime
 # model: the stack declares the edge once here; the graph only ever reads the encoded
@@ -142,6 +147,7 @@ def run_create(args: argparse.Namespace) -> int:
             manager="caddy",
             program=name,
             root=static_root or "dist",
+            spa=stack not in CONTENT_SITE_STACKS,
             description=description,
             requires=seeded_requires,
         )