feat: register searchindex output for mod-flexsearch lazy loading

mod-flexsearch's new lazyLoad option emits the search index via a
searchindex Hugo output format that must be attached to home outputs
at the theme level — the same wiring netlify and server use for
mod-csp. Module-level [outputs] config does not merge into consuming
sites; this is where the format gets activated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: markdumay

config/_default/hugo.toml

@@ -65,7 +65,7 @@ delimiter = ""
 
 # toml-docs-start outputs
 [outputs]
-home = ["HTML", "RSS", "REDIR", "netlify", "server"]
+home = ["HTML", "RSS", "REDIR", "netlify", "server", "searchindex"]
 # toml-docs-end outputs
 
 # toml-docs-start build