Phase 1: Session Metadata Foundation

[griffinmilsap]

Summary

This PR introduces the session-scoped metadata layer that the later control-plane
work builds on top of.

• Add session-backed GraphContext / GraphServer commands for connect,
  disconnect, register, clear, and snapshot operations.
• Introduce structured graph metadata models in graphmeta.py for component
  metadata, graph snapshots, and settings snapshot values.
• Teach the DAG and graph server to track edge ownership by session and clean up
  owned state when a session exits.
• Integrate metadata registration into runtime startup so ez.run(...) can
  expose component structure and initial settings to inspection tools.
• Add tests for session lifecycle, metadata collision handling, and topic
  behavior.

Scope

This PR is intentionally limited to session and metadata lifecycle. It does not
yet add the process-control plane, routed process requests, or profiling
streaming APIs. Those are stacked in the follow-on PR from
feature/process-control into feature/session-metadata.

Review Order

1. src/ezmsg/core/graphmeta.py
2. src/ezmsg/core/netprotocol.py
3. src/ezmsg/core/dag.py
4. src/ezmsg/core/graphcontext.py
5. src/ezmsg/core/graphserver.py
6. src/ezmsg/core/backend.py
7. tests/test_graph_session.py
8. tests/test_topics.py

Reviewer Checklist

• Session-owned edges are added and removed deterministically.
• Metadata registration is scoped to the session that created it.
• Component-address collisions are rejected cleanly.
• Graph snapshots are structured and IDE-friendly rather than dict-shaped
  protocol soup.
• Runtime startup and teardown keep session state coherent.

Suggested Validation

.venv/bin/pytest tests/test_graph_session.py tests/test_topics.py -q