fix(tui): preserve log rows during horizontal panning

[mingchuno]

Summary

• preserve a viewport row when horizontal clipping leaves a log record empty
• keep visible records and the reported line range stationary while panning
• cover readable and raw output, live following after an append, and a manually scrolled completed run

Validation

• pnpm verify passed (pnpm check, pnpm lint, and pnpm test)
• 193 tests passed, 2 skipped, and 0 failed

Limitations

• the live Codex and Copilot inspection tests were skipped
• validation used automated rendered-frame assertions; the supplied terminal recording was not replayed manually

Closes #18

Refs #18

[mingchuno]

Review of 3eddc1b

The implementation appears correct and scoped to the shared Logs viewer. One validation gap remains: the raw/readable coverage does not exercise records that render differently between those modes.