add khosra check, the place content errors are looked for

The server never fails on a bad bundle — it works around it and logs, which is easy
to miss (ADR-0029). This is the command that looks on purpose, and it exits non-zero
on anything that makes the site wrong rather than merely untidy.

Fatal: content the engine had to drop (unparseable frontmatter, a key two files
claim, an ignored slug or alias) and links that will 404 for a reader. Warnings:
no title, a figure with no alt text, a series where some members declare order and
others do not — that last one because unordered members sort last, so adding order
to one chapter silently moves every chapter that lacks it.

Two things this needed rather than invented. `Scan` and `Site` now *return* what they
worked around instead of only logging it: `ScanReport` and `Site.Problems`, with
`Scan` staying the logging wrapper so nothing else changed. And required-fields-per-type
is deliberately absent — `title` is the only field the engine requires today, so
checking more would mean inventing the type declaration that is still parked. Its
trigger stays where it was.

The link checker asks the same questions the resolver asks — bundle, alias, section
listing, file inside a bundle — because a checker that guesses differently from the
server is worse than no checker. Engine-owned paths are skipped: they are generated,
not authored.

It lives in internal/ext/check, so cmd/ stays wiring and the feature stays deletable.
Verified against the evidence site: clean before, and five findings across five fault
classes after I introduced them on purpose.

internal/content/site.go

@@ -14,6 +14,8 @@ type Site struct {
 	// keyByRoute maps a served path back to the identity it belongs to. Only renamed bundles appear: a
 	// bundle with no slug is served at its key, so route and key are the same string (ADR-0035).
 	keyByRoute map[string]string
+	// problems are what indexing worked around, kept for `check` rather than only logged.
+	problems []Problem
 	// renamed records keys that a slug moved away from, so the old path answers 404 instead of still working
 	// — the engine serves the new path only (ADR-0035), and an author who wants both writes an alias.
 	renamed map[string]bool
@@ -36,6 +38,16 @@ func NewSite(bundles []Bundle) *Site {
 	return s
 }
 
+// Problems lists what indexing worked around: a contested alias, a slug two variants disagree on, a slug
+// landing where something already answers. Each was also logged.
+func (s *Site) Problems() []Problem { return s.problems }
+
+// note records a problem and logs it, so the server says the same thing it always did.
+func (s *Site) note(path, detail string) {
+	s.problems = append(s.problems, Problem{path, detail})
+	slog.Error("content problem", "path", path, "detail", detail)
+}
+
 // indexRoutes resolves each key's served path from the slugs its variants declare.
 //
 // A slug renames the bundle in every language, so the variants have to agree: two declaring different slugs
@@ -59,13 +71,12 @@ func (s *Site) indexRoutes(bundles []Bundle) {
 			continue
 		}
 		if len(slugs) > 1 {
-			slog.Error("ignoring slug: variants of one bundle declare different ones",
-				"key", key, "slugs", sorted(slugs))
+			s.note(key, "slug ignored: variants declare different ones ("+strings.Join(sorted(slugs), ", ")+")")
 			continue
 		}
 		route := path.Join(path.Dir(key), sorted(slugs)[0])
 		if _, taken := s.byKeyLang[route+"\x00"+DefaultLang]; taken || s.keyByRoute[route] != "" {
-			slog.Error("ignoring slug: another bundle already answers there", "key", key, "route", route)
+			s.note(key, "slug ignored: "+route+" is already answered by another bundle")
 			continue
 		}
 		s.keyByRoute[route] = key
@@ -133,12 +144,12 @@ func (s *Site) indexAliases(bundles []Bundle) {
 		// ask the same question a request does.
 		if key, live := s.KeyFor(alias); live {
 			if _, isReal := s.byKeyLang[key+"\x00"+DefaultLang]; isReal {
-				slog.Error("ignoring alias that names a real bundle", "alias", alias, "claimed_by", keys)
+				s.note(alias, "alias ignored: a real bundle answers there, claimed by "+strings.Join(keys, ", "))
 				continue
 			}
 		}
 		if len(keys) > 1 {
-			slog.Error("ignoring alias claimed by more than one bundle", "alias", alias, "claimed_by", keys)
+			s.note(alias, "alias ignored: claimed by more than one bundle ("+strings.Join(keys, ", ")+")")
 			continue
 		}
 		s.aliases[alias] = keys[0]