add khosra demo, and give the site a front page

The demo writes a whole site root that exercises every feature: two languages with
a fallback, a series with ordered chapters, a gallery, a figure, an include,
extras, tags across sections, a slug with an alias, an undated page, a draft, a
template override, static files and site.yaml. It generates its filler rather than
copying stored files, because nothing in this repository is content (ADR-0011) — and
that makes it a test of the engine rather than a fixture: anything khosra can do
that the demo cannot express is a gap.

Two things found by generating and then serving it, which is the whole point:

`khosra check` reported the demo's own series as mixing ordered and unordered
members. It was right — the chapter bodies *described* `order: 10` while the
frontmatter never carried it. The checker caught its own author.

And `/` was a **404**. ADR-0008 leaves the root engine-owned, which is right, but
"engine-owned" was never given an answer, so a visitor to the site's own address got
nothing. The root now lists every bundle, newest first, paginated like any other
listing, and 404s only when nothing is published. A hand-written home page stays a
separate decision, recorded as such.

Verified end to end: 23 files written, 12 bundles, 12 derivatives, `check` clean,
and every URL the demo promises answers — including the alias redirecting, the draft
hidden, and the front page rendering through the site's *own* template override.

internal/web/web.go

@@ -74,7 +74,9 @@ func serveStatic(sub fs.FS) http.Handler {
 // A section is not a bundle, so this runs only after the bundle lookup misses. A page number past the
 // end is a 404 rather than an empty page, because an empty page is a URL that means nothing.
 func serveListing(w http.ResponseWriter, req *http.Request, site *content.Site, r *render.Renderer, res resolution) bool {
-	if res.key == "" || strings.Contains(res.key, "/") {
+	// An empty key is the site root, which lists everything. Anything with a slash in it is a bundle path that
+	// missed, not a section.
+	if strings.Contains(res.key, "/") {
 		return false
 	}
 	items := site.Run(content.Query{Section: res.key, Lang: res.lang})