[codex] Add Leaflet tile diagnostics overlay

[Komzpa]

Summary

• add debugTiles: true for L.TileLayer.Kothic
• draw a semi-transparent per-tile canvas overlay with z/x/y, tile URL, load/render status, and feature count
• keep the same diagnostics available from layer.getDebugMessages()
• surface JSON tile load failures through the tile callback so failed tiles can show diagnostics instead of staying silently gray
• cover loading, rendered, and HTTP error diagnostics in the Leaflet layer smoke test

Fixes #16.

Validation

• npm test
• git diff --check