feat: render server-generated terrain

[JJiang28]

Summary

• Fetches the terrain planes once from the new MapService.GetTerrain and renders ground, relief, vegetation, rivers and special resources as separate composited sprites. Previously every visible tile drew one grass texture and everything else drew fog.
• Splits tiles.ts, which baked each building into its own opaque tile texture and so could only ever draw a farm on grass — on desert it carried a patch of grassland with it. Buildings are now transparent sprites over whatever ground they occupy.
• Corrects the gameConfig defaults, which claimed a 128-wide map and a vision radius of 5 against a server serving 75 and 3.

Notes for reviewers

All layers share one texture geometry and therefore one anchor, deliberately. Per-layer heights would save ~0.4 MB, but each then needs its own anchor derived from its own CY / TEX_H; get one wrong and that layer sits ~26 px off while still looking almost right. Worse, hit-testing is pure math via pixelToHex and stays correct only while a sprite sits at exactly hexToPixel(col, row) — so "fixing" such an offset by nudging sprite.y would silently desync clicks from what is drawn. One geometry makes that unrepresentable.

Terrain outside vision is desaturated rather than blacked out. Nothing leaks: the server already filters entities by vision, so a fogged tile has no city or building to reveal.

getVisDist stays on Chebyshev distance on purpose — it mirrors domain.PointVisible server-side, and switching the client to true hex distance would make the fog ring disagree with which entities actually arrive.

Depends on city-io/backend#13, which adds GetTerrain. Merge that and deploy it first, or this build has no terrain to fetch.

proto/ here mirrors only the terrain additions. The army protos from backend #12 are not yet mirrored into this repo; that sync belongs with the armies frontend work rather than being smuggled in here.

Known follow-up

The renderer still tears down and rebuilds every visible tile on each ~3 s stream tick, and never frees tiles panned past. That predates this change, but terrain multiplies per-tile sprites, so the cost is now several times higher. A renderer restructure (pooling, real culling, diff updates, texture atlas) is the intended next change.

Test plan

• yarn check and yarn build clean
• yarn generate reproduces the committed generated code exactly
• Verified against a local backend: login, terrain renders under real city and building data, buildings composite onto their actual ground
• Fog desaturates terrain rather than hiding it; entities outside vision remain absent
• Construction overlay, territory borders, starvation border and tile selection all still render
• No console errors on load