[Feature] Dev-mode tracing visualizer (Flame-graph of message flows)

[pathosDev]

Size / Priority

• Size: M

Rationale

In production, traces go to Jaeger / Tempo / OTel-Collector. In dev, that setup is overkill. A bundled-in flame-graph / waterfall viewer for the in-process tracer surfaces span data without infra.

Design sketch

Built-in RecordingTracer already captures span data (src/tracing/RecordingTracer.ts). Add:

• HTTP endpoint: GET /tracing/spans returns recent spans.
• SPA: /tracing/visualizer renders flame-graph / waterfall.

Integration

• RecordingTracer: existing data source.
• Management endpoint: serves SPA + JSON.
• SPA: similar shape to [Feature] Live cluster visualizer (shipped in package) #204 (cluster visualizer) — small SPA bundled.

Out of scope / non-goals

• OTel SDK integration — already covered.
• Long-term storage — in-memory ring buffer only.

Test plan

1. Run workload; spans captured.
2. Open visualizer; flame-graph rendered.
3. Waterfall view alternative.
4. Bundle size ≤ 100KB.

Acceptance criteria

• /tracing/visualizer SPA.
• In-memory span buffer.
• Flame-graph + waterfall views.
• Documentation.
• Test suite.
• CHANGELOG entry.