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.
