feat: add FastAPI checkpoint continuation example

[rlippmann]

Summary

Adds a FastAPI checkpoint-continuation example.

The example demonstrates checkpoint continuation across stateless HTTP request boundaries. A travel-booking itinerary change enters a confirmation-required state, the host persists a checkpoint, and a later request restores that checkpoint into a fresh engine. Only a post-restore confirmation causes the booking change to be applied.

Why

The generic checkpoint-continuation examples demonstrate the concept in isolation.

This example shows the same enforcement point in a realistic web-service environment where requests are stateless and continuation state must survive between requests.

The example reinforces that:

• Context Compiler owns authoritative state and pending continuation state.
• The host owns persistence, routing, and booking mutation.
• Restoring authoritative state alone is insufficient to resume a pending confirmation flow.
• This is not prompt reinjection.

Example metadata

• Enforcement point: checkpoint continuation
• Domain: travel booking
• Technology/runtime: FastAPI
• Example type: reference integration
• Does this use directive drafter? should normally be no: no
• Does this derive state from model output? must be no for primary examples: no

Checklist

• Repository scaffolding / governance change
• One primary enforcement point
• Explicit authoritative state
• No model-derived state mutation
• Adversarial stub or equivalent test
• Observable runtime behavior change
• Domain vocabulary is natural
• Framework is secondary to the enforcement point
• Docs updated
• Tests/smoke checks added where practical

Notes

The example includes tests covering:

• pending confirmation creation
• checkpoint persistence
• checkpoint restore into a fresh engine
• confirmation after restore applies the change
• rejection after restore does not apply the change
• unrelated/adversarial input does not resolve the continuation
• authoritative-state-only restore is insufficient to resume the pending flow

The implementation uses an in-memory checkpoint store and booking store to keep the example focused on checkpoint continuation rather than infrastructure concerns.