fix: hydrate dashboard live updates incrementally

[douglasmonsky]

Summary

• fix Diagnostics-first navigation so Calls/Threads/Insights resume row hydration
• make Live refresh preserve existing rows, run the cached index refresh, and fetch only new rows from offset 0
• add focused dashboard runtime coverage for incremental live prepending

Validation

• PYTHONPATH=src .venv/bin/python -m ruff check .
• PYTHONPATH=src .venv/bin/python -m pytest
• for file in src/codex_usage_tracker/plugin_data/dashboard/dashboard*.js; do node --check ""; done
• PYTHONPATH=src .venv/bin/python scripts/check_release.py
• git diff --check
• Browser debug: Diagnostics-first route -> Calls hydrates; Live catches up to 25,112 calls without manual Refresh; no console warnings/errors