serve a bundle at its permalink

-site (or KHOSRA_SITE) opens the site root through content.OpenSite, so every
read keeps the os.Root guarantee. A path is a bundle key: /{section}/{slug}/
serves, the slashless form redirects permanently to it (ADR-0008), anything
unknown is 404. Render failure logs and returns a bare 500 rather than leaking a
template or filesystem detail.

internal/render holds goldmark plus the embedded reference theme (ADR-0026):
base.html with a redefinable "main" block, and one stylesheet inlined through
.Style. Serving it at an asset route would have been a second routing case for no
gain, and static serving belongs to a later entry.

Evidence beyond the tests: the binary against a real site root returns 200 with
<h1>About</h1> and the rendered body, 301 from /pages/about to /pages/about/, and
404 for /nope/. A Bengali variant is scanned but not yet reachable — that is the
next entry.

theme-contract.md gains a "Live today" section listing the six fields and two
named templates a theme may now rely on; the rest stays marked as shape.

internal/content/content.go

@@ -215,3 +215,30 @@ func dropCollisions(all []Bundle) []Bundle {
 	}
 	return kept
 }
+
+// Site is a set of bundles indexed for lookup by permalink key.
+type Site struct {
+	byKeyLang map[string]Bundle
+}
+
+// NewSite indexes bundles for lookup. Later variants of a key and language cannot occur, because Scan
+// drops ambiguity before this sees it.
+func NewSite(bundles []Bundle) *Site {
+	s := &Site{byKeyLang: make(map[string]Bundle, len(bundles))}
+	for _, b := range bundles {
+		s.byKeyLang[b.Key+"\x00"+b.Lang] = b
+	}
+	return s
+}
+
+// Lookup returns the default-locale variant of a key.
+//
+// Other languages are reachable only once language routing exists; until then a key means its default
+// variant (ADR-0021).
+func (s *Site) Lookup(key string) (Bundle, bool) {
+	b, ok := s.byKeyLang[key+"\x00"+DefaultLang]
+	return b, ok
+}
+
+// Len reports how many bundles the site holds.
+func (s *Site) Len() int { return len(s.byKeyLang) }