feat: render only ground the player can currently see

[JJiang28]

Summary

• The server now sends only the ground a player can currently see, so the client renders that and draws shroud everywhere else, instead of drawing the whole world desaturated.
• Makes fog a single source of truth: the client used to recompute Chebyshev distance from owned cities to decide what was fogged, which no longer agrees with the server now that armies also grant vision. Both the shroud and the vision-edge glow read the server's set.
• Surfaces terrain and any special resource on the selected tile.

Notes for reviewers

Applying an update clears the planes before writing the new set. Vision is ephemeral — it lasts only while a city or army is watching — so ground must actually go dark when the army that revealed it walks away. Merging updates instead of replacing would leave tiles lit forever and let the client slowly accumulate a map it is not entitled to.

The empty-tile message changed from "Beyond visibility range" to wording about nothing of yours watching, because that is now the actual mechanic: a tile is dark because it is unwatched, not because it is far away.

Known follow-up

The renderer still tears down and rebuilds every visible tile on each stream tick and never frees tiles panned past. That predates this work, but visibility changes now trigger redraws too, so it is worth doing before watchtowers make vision move more often. Restructure (pooling, real culling, diff updates, texture atlas) is the intended next change.

proto/ still mirrors only the terrain and vision additions; the army protos from backend #12 are not yet mirrored into this repo.

Test plan

• yarn check and yarn build clean
• yarn generate reproduces the committed generated code
• Verified against a local backend: only the ground around the player's city renders, the rest is shroud, and no console errors
• Selected tile reports terrain correctly (e.g. "51, 11 · Grassland")
• City card, building card, construction overlay and territory borders all still render inside the visible area