Add interactive Travel Agent Runtime explorer

[YingzuoLiu]

Summary

• add a standalone Vite/React explorer under interactive-demo/
• visualize an 11-step Router → Memory → Planner → Executor → Validator → Reducer → State flow
• support previous/next navigation, auto-play, component highlighting, state snapshots, and an execution trace
• add interview talking points and local run instructions
• link the explorer from the root README

Scope

The explorer is a deterministic presentation layer. It does not call real travel or booking APIs; the Python code remains the executable runtime reference.

Validation

• npm run build ✅
• git diff --check ✅

Python tests were not rerun because this environment does not have pytest installed; no Python files were changed.