timeline bug fixes plus tests, and one sliding sync fix for #1324

[eleboucher]

Description

sliding sync: subscription loading was only cleared when a RoomData envelope arrived. quiet rooms never send one, so the timeline stayed behind the spinner. it now clears after a full sync cycle.

timeline:

• row memo ignored permissions, so power level changes left stale redact/react/pin buttons
• merged rows share itemIndex -1, so jumping to an edit highlighted all of them
• new messages divider was dropped when the read marker sat on a filtered event
• reading on another device left the green line and unread chips behind
• failed backfill on an empty room showed placeholders with the retry unreachable

Fixes #1324

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-1540-sable.raspy-dream-bb1d.workers.dev	821f782	pr-1540	Fri, 31 Jul 2026 20:30:42 GMT