feat: inspect port and edge values

[philippjfr]

Description

This PR adds type-aware port tooltips and click-to-inspect support for handles and edges. A handle descriptor can now include a type, which renders with its label on hover. Handle and edge clicks emit events with their IDs and screen position, and ReactFlow.show_popup() renders a dismissible Panel overlay for application-specific value inspection.

The popup child is included in ReactFlow's child-model synchronization, and event callbacks are dispatched in the triggering document pass so callbacks can safely update child viewables.

Minimal example:

def on_handle_clicked(payload, flow):
    value = live_values[payload["node_id"]][payload["handle_id"]]
    flow.show_popup(pn.pane.Markdown(f"**Value:** {value!r}"), payload["position"])

flow.on("handle_clicked", on_handle_clicked)

How I tested:

• pixi run -e py312 test
• pixi run -e test-ui test-ui (48 passed, 2 reruns)
• pre-commit run on all changed files

AI Disclosure

Tool & Model: Kilo Code, kilo/openai/gpt-5.6-terra
Usage: I used it to implement the feature, investigate the child-view synchronization issue found during browser testing, write tests and documentation, and run validation.

• I have tested all AI-generated content in my PR.
• I take responsibility for all AI-generated content in my PR.

Checklist

• Tests added and are passing
• Added documentation