Keep the final-turn scroll range stable

[hamzamerzic]

Summary

• reserve the final-turn tail range before the latest user row enters the viewport, so reaching it cannot create a second-stage bottom
• keep the first touch and scroll tracing paths free of synchronous transcript layout reads
• update the owner scroll contract and focused regression coverage

Context

This is a focused follow-up to #197, #220, and #342. Those changes established the latest-turn spacer and reader-settlement contracts; this closes a later regression where the reserved range depended on viewport visibility and gesture-start tracing measured the full transcript.

Testing

• focused chat scroll contract tests (123 passing)
• frontend production build