Demote stackless Script error to non-paging

Stackless client 'Script error.' (opaque cross-origin/extension noise, 19 events/90d, 100% iOS Safari, NULL errorName+stack, app-side cause ruled out - no cross-origin scripts) now logs at warn instead of error via a stack-gated branch in classifyClientErrorLevel. Any report carrying a stack still pages at error. Deliberately NOT added to NETWORK_TRANSIENT_PATTERNS (shared with isNetworkTransientMessage which drives MARS-652 connection copy - would have misattributed a browser-extension throw as a dropped connection).