FE-1500: fix StrictMode playback disposal

[kube]

🌟 What is the purpose of this PR?

Simulation playback stalled in development builds: PlaybackProvider disposed its playback state machine in a useEffect cleanup, and React Strict Mode's setup, cleanup, setup replay left the handle permanently disposed, so play() and rAF ticks became no-ops while frames kept arriving. This removes the disposal effect.

🔗 Related links

• FE-1500

🔍 What does this change?

• Removes the dispose-on-cleanup effect from PlaybackProvider. Playback owns only in-memory state; its rAF loop and store subscriptions are cleaned up by their own effects. A comment at the site records why no disposal happens there.
• Adds a Strict Mode regression test that initializes a simulation, delivers frames across a rerender, drives two mocked animation frames, and asserts the frame index advanced.

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• do not affect the execution graph

🛡 What tests cover this?

• New: the react/playback/provider.test.tsx case "advances arriving frames when StrictMode replays mount effects".
• Existing: the rest of the PlaybackProvider suite.

❓ How to test this?

1. Checkout the branch and run yarn workspace @apps/petrinaut-website dev (the site mounts under StrictMode).
2. Open the local demo, switch to Simulate, and press Play.
3. Confirm the frame counter and playhead advance.

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Project	Deployment	Actions	Updated (UTC)
hash	Ready	Preview	Aug 29, 2026 2:04am
petrinaut	Ready	Preview	Aug 29, 2026 2:04am
petrinaut-docs	Ready	Preview	Aug 29, 2026 2:04am

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
hashdotdesign-tokens	Ignored	Preview	Aug 29, 2026 2:04am

[cursor]

PR Summary

Low Risk
Narrow React lifecycle fix with targeted test coverage; rAF cancellation remains in the existing playing effect.

Overview
Fixes simulation playback stalling in development when the app runs under React StrictMode. PlaybackProvider no longer calls playback.dispose() from a mount effect’s cleanup; that cleanup ran during StrictMode’s setup → cleanup → setup replay and left the same useState-held playback instance permanently disposed, so play() and rAF ticks stopped advancing the frame index while frames could still arrive.

A short comment documents that rAF and store subscriptions are torn down by their own effects, so the provider should not dispose the core playback handle on unmount replay. A new regression test wraps the provider in StrictMode, plays, rerenders with arriving frames, drives mocked animation frames, and expects currentFrameIndex to reach 2.

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