perf(render): optimize workspace navigation rendering

[nmrtist]

Summary

• Reduce workspace pan and zoom cost by rendering line and contour geometry within physical screen-pixel budgets during interaction.
• Filter contours against the plot viewport before deterministic sampling, and cache the resulting LOD so workspace navigation does not repeatedly scan the full contour geometry.
• Restore full-detail rendering after 200 ms of inactivity while keeping export rendering unchanged.
• Update plot viewports in place to avoid cloning materialized figures, and add render statistics plus a release-only multi-page NMR benchmark.

Validation

• cargo pr-check
• 13-page release benchmark: Full median 143.75 ms; Interactive median 8.87 ms, with no full contour scans in measured interactive frames.