fix: stabilize guided demo event detection

[btcneves]

Summary

• Add short polling for mempool entry detection and ZMQ rawtx/rawblock event-store confirmation.
• Preserve honest async fallback messages when monitor events do not arrive within the timeout.
• Fix dashboard hero layout so Proof Report and observability cards no longer overlap the live node console.
• Add focused tests for NDJSON ZMQ lookup and transient mempool RPC misses.

Validation

• npm run lint
• npm run typecheck
• npm run format:check
• npx vite build --outDir /tmp/nodescope-dashboard-fix-dist --emptyOutDir
• docker compose config
• docker compose up -d --build
• docker compose run --rm nodescope-api-test
• make smoke
• Full Guided Demo via /demo/reset, /demo/run, /demo/status: all 14 steps passed; Proof Report had no warnings.
• Playwright dashboard geometry check: hero cards do not overlap .lab-window.