Ability engine, cooldowns, counters and the six slots

Phase 1 of #147/#148. One ability pipeline in uo-combat (docs/builds.md § 3.2) driven by AbilityDef: costs (stamina, mana, reagents), wind-up and telegraph, shapes on tiles, effects, cooldowns per slot. No dice (§ 4.5): every chance stat (crit, block, hit, fizzle, AI parry) becomes a counter that fires at 100; damage rolls within ±10 %. Port #145's techniques onto it. Protocol Ability { slot, target }, SetLoadout; AbilityUsed generalises Telegraph; counters in EntityState.

Done when every #145 test passes through the new pipeline, no chance roll is left in combat resolution, and Q/W/E/R/D/F work offline and online.