Feat: Add Viewport Renderer

[Azraeel]

This PR improves performance on large, dense maps by virtualizing the most expensive SVG layers. Burg labels/icons and routes now render through a viewport-aware renderer, so the live DOM stays much smaller during pan and zoom while preserving normal SVG crispness.

What Changed

• Added a ViewportRenderer for burg labels, burg icons, anchors, and routes.
• Renders only visible or high-priority items at normal map scale, with level-of-detail behavior while zooming.
• Keeps editor and overview workflows working by force-rendering selected burgs/routes when needed.
• Ensures exported SVG/PNG clones still include the full burg and route layers.
• Optimized scale bar redraws to avoid rebuilding its SVG content on every zoom frame.
• Fixed a legacy map migration crash when old saves have burg data but no burg icon groups.

Why

Large maps with many cities, labels, nations, and routes were becoming sluggish because pan/zoom had to move and repaint thousands of live SVG nodes. This keeps the interactive map lightweight while keeping exports and editing behavior complete.

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[Azraeel]

This helps significantly on 100k point maps with lots of burgs, rivers, and labels.
Kills most of the oppressive UI Lag on huge maps too.

Performance Notes

Measured against origin/master on a dense generated map:

Metric	master	viewport-renderer	Change
Burg label DOM nodes	1,202	324	-73.0%
Burg icon DOM nodes	1,202	324	-73.0%
Route path DOM nodes	1,072	463	-56.8%
Exported burg labels	1,202	1,202	full export preserved
Exported burg icons	1,202	1,202	full export preserved
Exported routes	1,072	1,072	full export preserved

Chrome trace over scripted pan/zoom sequence:

Trace work	master	viewport-renderer	Change
Layout	292.43ms	192.50ms	-34.2%
PrePaint	58.24ms	32.79ms	-43.7%
Paint	124.06ms	106.18ms	-14.4%
Layout + PrePaint + Paint	474.73ms	331.47ms	-30.2%

[Azgaar]

Hello and thanks for the contribution! I like the concept, but the code is really messy and unpolished. If we rethink it into a minimal system, then we can try to go with it.

[Azraeel]

sure i can do some significant polish !

[Azgaar]

I mean we need to rethink the concept. E.g. for burg labels/icon we already have a system to hide them based on zoom level, so we can omit them from DOM as well. Can do the same for routes. You solution is overcomplicated, like on zoom 1 I get 812 burgs of group town generated, all of them as hidden on that zoom, but I see 188 of them in DOM. I would prefer a ViewportRenderer to completely replace the old invokeActiveZooming function and zoomRaf concept to actually remove/add elements to DOM rather then just hide/show on the parent level.

There is also a dependency on labels positions that we currently keep in svg (not in pack data), so #1313 should be completed first to not lose the data on dom recreation.

[Azraeel]

ahhh thats makes sense!
I can work on that if we could communicate more deeply on the topic!

[Azgaar]

Yes, would need to start with a proper planning, PRD creation, then architecture scheme and only then to development.

[Azgaar]

Drafted an AI PRD for the change:
viewport_rendering_prd_Version3.md

It still requires a lot of thinking as it can be done more elegantly, but the idea is clear.