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/render/chrome.go

@@ -24,6 +24,7 @@ var chrome = map[string]map[string]string{
 	"position":     {"en": "%s of %s", "bn": "%s / %s"},
 	"extras":       {"en": "Extras", "bn": "অতিরিক্ত"},
 	"back-to-page": {"en": "Back to the page", "bn": "পৃষ্ঠায় ফিরুন"},
+	"everything":   {"en": "Everything", "bn": "সবকিছু"},
 	"first":        {"en": "First", "bn": "প্রথম"},
 	"last":         {"en": "Last", "bn": "শেষ"},
 }