Fix chat tail-follow jitter and layout-safe geometry

[YeungKC]

Summary

• Keep the old bottom position during tail-follow appends so new messages animate from the previous bottom instead of snapping first.
• Let the clamping viewport skip one automatic bottom correction when the chat coordinator has a tail-follow anchor.
• Treat not-yet-laid-out message geometry as unavailable for the current frame, preventing gray error frames during chat rebuilds.
• Add regression coverage for bottom appends and the existing scroll restore paths.

Testing

• flutter test test/ui/home/chat/chat_scroll_coordinator_test.dart
• dart analyze lib/ui/home/chat/chat_scroll_coordinator.dart
• dart analyze on the affected chat scroll and clamping viewport files
• Profile smoke check with chat jump/scroll trace enabled