fix(composer): scope scheduled send state to the room composer

[eleboucher]

Description

scheduled send state is a per-room atom so a thread composer in the same room was reading and writing the room composer's scheduled time and delay id.

thread composers read null now and their setters do nothing. scheduling stays room-only.

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[github-actions]

Deploying with    Cloudflare Workers

Status	Preview URL	Commit	Alias	Updated (UTC)
✅ Deployment successful!	https://pr-1565-sable.raspy-dream-bb1d.workers.dev	3a84eda	pr-1565	Sat, 01 Aug 2026 15:41:35 GMT