serve a declared slug as an address, leaving the key alone

The human chose the second option: a route sits beside the key rather than
replacing it. So `slug` renames what a bundle is served at, in every language, and
identity stays derived from the path — which is exactly what keeps ADR-0033 intact,
since series membership is the directory. A series landing page can now be renamed
without orphaning its chapters, and there is a test that says so.

`Site` resolves routes at index time, because only it can see whether every variant
agrees. Disagreement is dropped rather than resolved, as is a slug landing where
another bundle already answers — the same rule colliding keys and contested aliases
already follow. The key a slug moved away from stops answering, so the old address
does not quietly keep working.

Two bugs surfaced doing this, both older than this change:

An alias naming its own bundle's former key was rejected as "an alias that names a
real bundle" — which made rename-plus-alias, the entire point of ADR-0008's alias
mechanism, impossible. The check now asks what a request asks: is anything actually
served there.

Aliases were counted per declaring *file*, so a bundle whose two language variants
both listed the same alias looked like two rival claimants and lost the alias. It is
a set of keys now. This one only appears with translated content, which is why no
fixture had caught it since entry 4 — the real binary did, on the first multilingual
rename.

internal/content/content.go

@@ -43,6 +43,12 @@ type Bundle struct {
 	// Aliases are paths that must keep resolving to this bundle, each redirecting to its canonical URL
 	// (ADR-0008). Additive only: an alias is a promise never withdrawn.
 	Aliases []string
+	// Slug is a hand-chosen final path segment, empty unless frontmatter declares one. It renames the
+	// bundle's address in every language (ADR-0035) and never its Key, which stays the identity.
+	Slug string
+	// Route is the path this bundle is served at: its Key, unless a slug renamed the last segment. Set by
+	// NewSite, which is the only place that can see whether every variant agrees.
+	Route string
 	// Order is this bundle's position in the series it is nested under, zero when frontmatter omits it.
 	// The convention is sparse (10, 20, 30), so zero is not a position: an unordered member sorts by name
 	// after every ordered one (ADR-0033).
@@ -131,6 +137,13 @@ func Parse(name string, data []byte) (Bundle, error) {
 	delete(b.Extra, "tags")
 	b.Order = asInt(b.Extra["order"])
 	delete(b.Extra, "order")
+	if slug, isStr := b.Extra["slug"].(string); isStr {
+		// One segment, normalised like every other identifier (ADR-0015). Slashes would let a slug move the
+		// bundle to another section, which is a move, not a rename.
+		b.Slug = Normalise(strings.Trim(strings.TrimSpace(slug), "/"))
+	}
+	delete(b.Extra, "slug")
+	b.Route = b.Key
 	return b, nil
 }