fix(08-09): match wristband OAuth byte contract to live-recorded PHP

Recording the full mcp-lifecycle fixture against real isolated PHP
(08-09-PLAN.md Task 2) uncovered three byte-level gaps between wristband's
assumed contract and actual production PHP behavior:

- Every explicit "Cache-Control: no-store" PHP sets is actually delivered
  as "no-store, private" (Laravel's session-cookie default merges "private"
  onto any explicit value); wristband's own default for unheadered JSON
  error responses is "no-cache, private" (matching the house convention
  already used elsewhere), not empty.
- PHP's redirect responses (authorize success and every error redirect)
  render Symfony's default HTML redirect body with Content-Type
  "text/html; charset=utf-8"; Go's bare 302 with no body never matched.
  wristband/redirect_html.go ports that exact byte template, including
  PHP's htmlspecialchars(ENT_QUOTES) escaping (Go's html.EscapeString uses
  different quote entities).

tide/normalize.go: isIDKey now also masks "_ids" plural array fields
(e.g. collection_ids), a latent parity-corpus gap no prior fixture had
exercised with a literal, non-empty, non-placeholder array value.

wristband/register.go

@@ -166,11 +166,11 @@ func (s *Server) Register(w http.ResponseWriter, r *http.Request) {
 		zero := int64(0)
 		resp.ClientSecretExpiresAt = &zero
 	}
-	writeExactJSON(w, http.StatusCreated, resp, map[string]string{"Cache-Control": "no-store"})
+	writeExactJSON(w, http.StatusCreated, resp, map[string]string{"Cache-Control": "no-store, private"})
 }
 
 func writeRegisterError(w http.ResponseWriter, status int, code, description string) {
-	writeExactJSON(w, status, rfcErrorBody{Error: code, ErrorDescription: description}, map[string]string{"Cache-Control": "no-store"})
+	writeExactJSON(w, status, rfcErrorBody{Error: code, ErrorDescription: description}, map[string]string{"Cache-Control": "no-store, private"})
 }
 
 // isJSONContentType mirrors Laravel's Request::isJson(): the Content-Type