feat: gate terrain behind live vision and let armies see

[JJiang28]

Summary

• Terrain was the only thing in the game not filtered by vision — cities, buildings, armies and tiles all go through the fog filters, so any authenticated client could read the entire world. GetTerrain now returns only currently visible ground.
• Replaces the cities-only vision helpers with a Watcher box so any entity can grant sight, and makes armies see. They previously could not: they were merely filtered by city vision, so marching one across the map revealed nothing and a player's view never changed after founding.
• StreamState carries a fresh VisibleTerrain whenever the visible set moves.

Notes for reviewers

Vision is ephemeral by design. Ground is lit only while a city or army is actually watching it and goes dark when that thing leaves. There is deliberately no remembered-terrain model: holding sight of the map is meant to be maintained, which is the groundwork for buildable watchtowers and fortresses on captured neutral tiles — vision you build, and can lose.

VisibleTerrain is always sent whole, never as a reveal-only delta. Because vision is ephemeral, tiles leave the set as readily as they enter it. Sending only reveals would require the client and server to agree on concealments too, and any drift leaves ground permanently lit that should be dark. Replacing the set outright makes that unrepresentable.

Recomputation is skipped on stream ticks that cannot have moved vision (resource-only updates), since it costs a query per subscriber and most ticks change nothing.

Follow-up, not in this PR

Watchtower and fortress buildings, and conquering neutral tiles, are deliberately out of scope. A fortress that grants defence is meaningless until combat exists — armies can currently move and merge but not fight.

VisionRadius is still 3. With ephemeral vision and no remembered terrain, that is a fairly small window on a 75×75 map; it may want raising once watchtowers land and there is a way to extend sight deliberately.

Test plan

• go build ./..., go vet ./... and gofmt -l internal/ cmd/ clean
• buf lint clean; buf generate reproduces the committed output
• Boots against a local Postgres and serves; world generates and towns seed as before
• Verified in a browser: only the ground around the player's city renders, everything else is shroud
• Tile detail reports the correct terrain for a visible tile
• No console errors on the client against this backend