feat: add randomized combat and frontier defenses

[prayujt]

Outcome

• replaces deterministic casualty carry with battle-seeded per-unit casualty rolls, preserving expected losses while allowing upset outcomes
• adds upgradeable standalone watchtowers and forts on neutral land within a configurable settlement placement radius
• requires one of the builders live armies to occupy the target tile when frontier construction begins
• adds configurable, level-scaled watchtower vision and fort, town-center, and city-center defense bonuses
• extends persistence, streaming, visibility, RPC contracts, generated clients, and game config for standalone structures

Initial balance

• placement radius: 10 tiles from an owned settlement footprint
• watchtower vision: 3 to 12 tiles across levels 1 to 10
• fort defense: 5% to 50%
• town-center defense: 10% to 55%
• city-center defense: 15% to 60%

Eligible center and fort bonuses stack and use completed building levels. All values live in backend balance tables and are exposed through GetGameConfig.

Verification

• go test ./...
• go vet ./...
• git diff --check