Allow creating graphs with parameterized Node and Edge instances

[philippjfr]

Summary

This PR adds first-class instance-based graph elements to Panel-ReactFlow by introducing Node and Edge base classes and wiring them into the core sync/event/editor pipeline.

Users can now pass Node/Edge objects directly (not just dicts), define reusable subclasses with parameters, and build object-oriented graph workflows with automatic two-way data synchronization.

What’s included

• Added new base classes:
  • Node (with shared node fields, to_dict(), optional __panel__, editor hook, on_* hooks)
  • Edge (with shared edge fields, to_dict(), editor hook, on_* hooks)
• Exported Node and Edge in package API.
• Updated ReactFlow to accept/retain Node and Edge instances in nodes/edges.
• Serialization of instances happens only in _process_param_change.
• Added per-instance event hook dispatch for nodes/edges (on_event, on_add, on_delete, etc.).
• Added per-instance editor support (node.editor(...), edge.editor(...)).
• Added automatic two-way parameter <-> data behavior for subclass params with precedence >= 0:
  • included in data
  • included in auto-generated schema (when no explicit schema provided)
  • parameter changes auto-trigger data patch + frontend sync
  • incoming data patches/sync update matching instance parameters
• Added watcher lifecycle management and cleanup when nodes/edges are removed/replaced.

Docs

• Updated guides to document:
  • class-based nodes/edges
  • __panel__ usage
  • parameter/data sync semantics
  • precedence rule (>= 0) for data/schema inclusion
  • event behavior for parameter-driven patches
• Added/updated examples, including complex instance-based workflows and a Three.js instance-based variant.

Tests

Added/extended tests to cover:

• acceptance of Node/Edge instances
• sync and patch behavior updating instance state directly
• hook dispatch behavior
• per-instance editor resolution
• parameter inclusion in data + schema generation
• automatic param-change patching
• watcher cleanup on delete
• bokeh model children initialization for view/editor child trees

All relevant tests pass locally.

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Test plan

• pytest tests/test_api.py tests/test_core.py -q
• Verify examples compile:
  • python -m py_compile examples/node_edge_instances.py
  • python -m py_compile examples/threejs_viewer_instances.py
• Manual UI smoke test (recommended):
  • Run example app(s), modify parameter-bound widgets, confirm graph data and viewer update immediately
  • Add/remove instance nodes and edges, confirm no stale behavior or leaks

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Backward compatibility

• Existing dict / NodeSpec / EdgeSpec usage is preserved.
• New instance-based APIs are additive and optional.