feat(auth): X-Bn-Verified-Captcha trusted header + CaptchaVerified helper

The host verifies (and consumes) a request's cap-token against its
stateful captcha server before wasm dispatch and stamps this header;
guests enforce captcha by checking CaptchaVerified, fail closed.
Included in AllTrustedHeaders so the host's strip-then-stamp loop
makes it unforgeable, like the identity headers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

auth/trustedheaders.go

@@ -36,6 +36,13 @@ const (
 	HeaderVerifiedPublicUserID   = "X-Bn-Verified-Public-User-Id"
 	HeaderVerifiedPublicUsername = "X-Bn-Verified-Public-Username"
 	HeaderVerifiedPublicEmail    = "X-Bn-Verified-Public-Email"
+
+	// Captcha proof. The host verifies (and consumes — replay-protected) the
+	// request's cap-token against the instance captcha server before dispatch
+	// and stamps "1" on success. A guest cannot hold the host's stateful
+	// verifier, so this header is the ONLY way to enforce captcha on a plugin
+	// HTTP endpoint; render the widget with the ninjatpl captcha tag.
+	HeaderVerifiedCaptcha = "X-Bn-Verified-Captcha"
 )
 
 // AllTrustedHeaders lists every trusted identity header, canonical form. The
@@ -49,9 +56,16 @@ func AllTrustedHeaders() []string {
 		HeaderVerifiedPublicUserID,
 		HeaderVerifiedPublicUsername,
 		HeaderVerifiedPublicEmail,
+		HeaderVerifiedCaptcha,
 	}
 }
 
+// CaptchaVerified reports whether the host verified a captcha token for this
+// request. Fail closed: enforcing handlers must reject when this is false.
+func CaptchaVerified(h http.Header) bool {
+	return h.Get(HeaderVerifiedCaptcha) == "1"
+}
+
 // ContextFromTrustedHeaders rebuilds the request's auth context from the host's
 // verified identity headers. Trust model: see the const block above — these
 // headers are host-controlled, so no token parsing or signature check happens

auth/trustedheaders_test.go

@@ -80,3 +80,27 @@ func TestTrustedHeaderMiddleware(t *testing.T) {
 		t.Fatalf("middleware did not install principal: %+v", got)
 	}
 }
+
+func TestCaptchaVerifiedReadsTrustedHeader(t *testing.T) {
+	h := http.Header{}
+	if CaptchaVerified(h) {
+		t.Fatal("CaptchaVerified() = true with no header, want false")
+	}
+	h.Set(HeaderVerifiedCaptcha, "1")
+	if !CaptchaVerified(h) {
+		t.Fatal("CaptchaVerified() = false with host-stamped header, want true")
+	}
+	h.Set(HeaderVerifiedCaptcha, "true")
+	if CaptchaVerified(h) {
+		t.Fatal(`CaptchaVerified() = true for non-"1" value, want false`)
+	}
+}
+
+func TestAllTrustedHeadersIncludesCaptcha(t *testing.T) {
+	for _, h := range AllTrustedHeaders() {
+		if h == HeaderVerifiedCaptcha {
+			return
+		}
+	}
+	t.Fatalf("AllTrustedHeaders() missing %s — hosts strip-then-stamp from this list, so omitting it makes the header client-forgeable", HeaderVerifiedCaptcha)
+}