fix(mobile): preserve channel scroll anchor on updates

[BrianInAz]

Summary

• preserve the stable visible message anchor when a live message is prepended to the reversed channel list
• keep follow mode off until the user explicitly chooses Jump to Latest
• strengthen the tall-message regression with geometry and hit-testing assertions

TDD evidence

• Red on origin/main ef72743: visible tall-message top shifted from 142 px to 44 px after append
• Green targeted regression: 1 passed
• Full ChannelDetailPage file: 61 passed
• mobile-check: passed
• full mobile suite: 1,098 passed, 1 intentional skip, 0 failed

Scope

This is the separately authorized remediation for the pre-existing ChannelDetailPage failure. It contains no presence, Sprig, relay, Desktop, CLI, or identity changes.

Beads: ios-buzz-59e.7

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