[Perf]: pan and zoom still stall when all heavy nodes are visible

[siarheihuzarevich]

Follow-up from #266 and the latest user feedback:
#266 (comment)

Summary

v18.2.0 improved panning when only a fraction of heavy nodes are in the viewport, but interaction still degrades noticeably when all expensive nodes are visible at once (for example after fitToScreen).

Expected

Pan/zoom should remain responsive even when the initial camera exposes the full heavy-card scene, or we should provide a documented strategy/API that keeps startup and interaction smooth in that state.

Relationship to #266

This is related to the broader "only render visible elements" request, but it is actionable enough to track separately because the latest report confirms the remaining bottleneck after the v18.2.0 improvements.

Investigation ideas

• Benchmark pointer-down, pan, and zoom cost with 50 heavy nodes / 50 connections.
• Separate DOM/render cost from relayering/selection cost.
• Evaluate whether viewport-only rendering, occlusion culling, or stronger level-of-detail behavior is required.