test: add an interactive custom projection app - #10744

Confidence Score: 5/5

The PR appears safe to merge based on the reviewed changes.

Summary

This PR adds an interactive app for exploring four custom projections. It includes country, airport, and graticule layers; runtime projection switching; reusable projection configurations; and setup documentation.

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  Selector[Projection selector] --> Config[Projection configuration]
  Config --> View[CustomProjectionView]
  Data[Countries, airports, graticules] --> Layers[GeoJsonLayer instances]
  View --> Deck[Deck]
  Layers --> Deck
  Deck --> Hover[Input-coordinate hover display]

_{Reviews (3) · Last reviewed commit: "test: use public registry URLs in lockfi..."}

Coverage is 82.971% — x/alt-proj-test-app into x/alt-proj-aggregation-plumbing. No base build found for x/alt-proj-aggregation-plumbing.