MLS group state recovery + new-device join

[codebestia]

Handle a newly-linked device joining existing groups: it must be added via proposal/commit to receive future epochs; past group messages it has no key for surface as unavailable. Document the "no history for new device" property (unless backed by the deferred recovery service).

Acceptance criteria:

• New device joins groups via commit and gets future messages
• Pre-join messages shown as unavailable, not errors
• Behavior documented for users

[DeveloperEmmy]

@DeveloperEmmy has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello @maintainer, I’ve checked the requirements, and I’m confident I can implement a proper solution that matches the acceptance criteria. Kindly assign this issue to me. I will deliver in no time.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @DeveloperEmmy to this issue.

[Olorunfemi20]

@Olorunfemi20 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello! This issue aligns well with my experience, and I'd like to work on it. I've already reviewed the expected behavior and have a good understanding of the implementation required. I'm confident I can deliver a polished solution that meets the project's standards.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Olorunfemi20 to this issue.

[privyy-bit]

@privyy-bit has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

i would like to work on this issue kindly assign me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @privyy-bit to this issue.

[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello, I'd like to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Caneryy to this issue.

[drips-wave]

Congratulations, @Olorunfemi20! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Olorunfemi20: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊