[Feature] Live cluster visualizer (shipped in package)

[pathosDev]

Size / Priority

• Size: L — UI work + framework integration.

Rationale

Operators monitoring a live cluster benefit from a visual topology: which nodes, which shards, what's the gossip flow, what messages are slow. Akka had this via separate tools; for a self-contained "open browser → see cluster" experience, ship a small SPA in the package.

Design sketch

• Backend: extend management HTTP endpoint with /cluster/visualizer/* routes.
• WebSocket: stream live cluster events (member up/down, shard moves, gossip ticks, hot-actor mailbox depths).
• SPA: small React/Svelte/Vue bundle served from /cluster/visualizer. ~200KB gzipped target.
• Views:
  • Topology graph (nodes + connections).
  • Shard distribution heatmap.
  • Recent gossip / membership changes.
  • Per-actor hot-list (highest mailbox depth, slowest handlers).
  • Cluster events timeline.

Integration

• Management HTTP endpoint (Cluster-management HTTP endpoints — extended (shards, leave-and-shutdown, metrics) #56): extend.
• ClusterEvents: stream over WebSocket.
• Metrics: pull aggregated stats per second.
• Bundle: build-time generation; ships as static assets.

Out of scope / non-goals

• Production-grade monitoring — that's Prometheus/Grafana. This is dev/diagnostics.
• Auth — relies on existing management-endpoint auth.

Open design questions

1. Framework choice for SPA: React (familiar), Svelte (smaller bundle), or vanilla. Recommend Svelte.
2. Topology layout: force-directed vs ring vs custom. Recommend force-directed.
3. Update rate: 1Hz vs higher. Recommend 1Hz default.

Test plan

1. Open /cluster/visualizer → topology renders.
2. Membership change reflected within 1s.
3. Shard move animates.
4. Hot-actor list updates.
5. Bundle size ≤ 200KB gzipped.
6. Works without external CDN (everything bundled).

Acceptance criteria

• /cluster/visualizer SPA + supporting endpoints.
• Live event stream.
• Topology + heatmap + timeline views.
• Documentation.
• Test suite.
• CHANGELOG entry under "New: Cluster visualizer SPA".

Pre-implementation checklist

• Framework choice.
• Bundle size budget agreed.
• Build pipeline integrated.