perf(nmr): optimize 2D phase previews

[nmrtist]

Summary

• coalesce continuous 2D phase inputs while keeping the latest complete plot visible
• reuse cached field grids and fixed contour thresholds during phase previews
• stream NMR component planes to reduce repeated validation and sampling work
• preserve dataset identity across phase interactions and surface preview state in contour readouts
• document the updated preview behavior in English and Simplified Chinese

Validation

• cargo pr-check
• npm run build in docs/