Document Pi's structured-output timing caveat

Record that Chopin's Pi wrapper answers its synthetic result tool one
event-loop turn after Pi reports the call, why that holds in the pinned
@ai-sdk/harness-pi 1.0.128, and that the Pi contract suite must run
before bumping it.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: bun run ci passed: dprint, oxlint, tokens
User-preference: Record third-party harness limitations as documented caveats rather than patching dependencies
Co-authored-by: Alex Lavaee <lavaman131@github.com>