Add temporary hit points as a separate damage buffer

Temp HP absorbs damage before current HP, cannot be healed, and
does not stack (higher value wins). Displayed as cyan +N after
current HP with a Shield button in the HP adjustment popover.
Column space is reserved across all rows only when any combatant
has temp HP. Concentration pulse fires on any damage, including
damage fully absorbed by temp HP.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/application/src/index.ts

@@ -21,5 +21,6 @@ export { rollInitiativeUseCase } from "./roll-initiative-use-case.js";
 export { setAcUseCase } from "./set-ac-use-case.js";
 export { setHpUseCase } from "./set-hp-use-case.js";
 export { setInitiativeUseCase } from "./set-initiative-use-case.js";
+export { setTempHpUseCase } from "./set-temp-hp-use-case.js";
 export { toggleConcentrationUseCase } from "./toggle-concentration-use-case.js";
 export { toggleConditionUseCase } from "./toggle-condition-use-case.js";

packages/application/src/set-temp-hp-use-case.ts

@@ -0,0 +1,24 @@
+import {
+	type CombatantId,
+	type DomainError,
+	type DomainEvent,
+	isDomainError,
+	setTempHp,
+} from "@initiative/domain";
+import type { EncounterStore } from "./ports.js";
+
+export function setTempHpUseCase(
+	store: EncounterStore,
+	combatantId: CombatantId,
+	tempHp: number | undefined,
+): DomainEvent[] | DomainError {
+	const encounter = store.get();
+	const result = setTempHp(encounter, combatantId, tempHp);
+
+	if (isDomainError(result)) {
+		return result;
+	}
+
+	store.save(result.encounter);
+	return result.events;
+}