perf(cli): memoize console preview rows

[codeaholicguy]

Summary

• extract a pure flattened preview row-model helper
• memoize preview rows by messages and slice the viewport from that model
• preserve the existing message-based viewport helper while avoiding duplicate scans in PreviewPane
• add a deterministic Ink rerender regression test for scroll-only updates

Validation

• npm test --workspace packages/cli -- src/tests/tui/console/PreviewPane.test.ts (11 passed)
• npm test --workspace packages/cli (960 passed)
• npm run lint --workspace packages/cli (exit 0; 5 existing unrelated warnings)
• npm run build --workspace packages/cli (exit 0)

Risks

• None known; viewport, continuation indicator, append-offset adjustment, multiline, timestamp, and scrolling behavior continue through the existing tested row-slicing path.