Add min/max hit point variants for D&D creatures

A D&D stat block opened for a combatant now offers a Min/Avg/Max toggle
below the Hit Points line, deriving both ends from the Hit Dice formula
(3d6 + 6 -> 9 / 16 / 24). The switch is applied as a delta, so manual HP
edits and damage already taken survive it. Unlike the PF2e weak/elite
adjustment this is a convenience tool, not a rules mechanic: nothing but
HP changes and the combatant is not renamed. The toggle is hidden when
the HP field carries prose instead of a dice pool.

Also extracts a SegmentedControl primitive. Game system, theme and the
PF2e weak/elite toggle were three hand-rolled copies of the same markup,
which is why the D&D toggle first came out chunkier and in the wrong
accent color. All four now share one definition, and segments expose
aria-pressed instead of signalling the active state by color alone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/web/src/hooks/__tests__/use-encounter-hp-variant.test.ts

@@ -0,0 +1,95 @@
+import type { HpRange, HpVariant } from "@initiative/domain";
+import {
+	combatantId,
+	creatureId,
+	EMPTY_UNDO_REDO_STATE,
+} from "@initiative/domain";
+import { describe, expect, it } from "vitest";
+import { type EncounterState, encounterReducer } from "../use-encounter.js";
+
+// An ogre with 3d6 + 6 hit points.
+const RANGE: HpRange = { min: 9, average: 16, max: 24 };
+
+function stateWithCreature(
+	maxHp: number,
+	currentHp: number,
+	hpVariant?: HpVariant,
+): EncounterState {
+	return {
+		encounter: {
+			combatants: [
+				{
+					id: combatantId("c-1"),
+					name: "Ogre",
+					maxHp,
+					currentHp,
+					creatureId: creatureId("mm:ogre"),
+					...(hpVariant !== undefined && { hpVariant }),
+				},
+			],
+			activeIndex: 0,
+			roundNumber: 1,
+		},
+		undoRedoState: EMPTY_UNDO_REDO_STATE,
+		events: [],
+		nextId: 1,
+		lastCreatureId: null,
+	};
+}
+
+function setVariant(state: EncounterState, variant: HpVariant | undefined) {
+	return encounterReducer(state, {
+		type: "set-hp-variant",
+		id: combatantId("c-1"),
+		variant,
+		range: RANGE,
+	});
+}
+
+describe("set-hp-variant", () => {
+	it("raises max HP to the maximum roll and stores the variant", () => {
+		const next = setVariant(stateWithCreature(16, 16), "max");
+		const c = next.encounter.combatants[0];
+		expect(c.maxHp).toBe(24);
+		expect(c.currentHp).toBe(24);
+		expect(c.hpVariant).toBe("max");
+	});
+
+	it("lowers max HP to the minimum roll", () => {
+		const next = setVariant(stateWithCreature(16, 16), "min");
+		const c = next.encounter.combatants[0];
+		expect(c.maxHp).toBe(9);
+		expect(c.hpVariant).toBe("min");
+	});
+
+	it("returns to the average and clears the variant", () => {
+		const next = setVariant(stateWithCreature(24, 24, "max"), undefined);
+		const c = next.encounter.combatants[0];
+		expect(c.maxHp).toBe(16);
+		expect(c.hpVariant).toBeUndefined();
+	});
+
+	it("does not rename the combatant", () => {
+		const next = setVariant(stateWithCreature(16, 16), "max");
+		expect(next.encounter.combatants[0].name).toBe("Ogre");
+	});
+
+	it("is undoable", () => {
+		const start = stateWithCreature(16, 16);
+		const next = setVariant(start, "max");
+		const undone = encounterReducer(next, { type: "undo" });
+		expect(undone.encounter.combatants[0].maxHp).toBe(16);
+		expect(undone.encounter.combatants[0].hpVariant).toBeUndefined();
+	});
+
+	it("leaves state untouched for an unknown combatant", () => {
+		const start = stateWithCreature(16, 16);
+		const next = encounterReducer(start, {
+			type: "set-hp-variant",
+			id: combatantId("c-99"),
+			variant: "max",
+			range: RANGE,
+		});
+		expect(next).toBe(start);
+	});
+});

apps/web/src/hooks/use-encounter.ts

@@ -15,6 +15,7 @@ import {
 	setConditionValueUseCase,
 	setCrUseCase,
 	setHpUseCase,
+	setHpVariantUseCase,
 	setInitiativeUseCase,
 	setSideUseCase,
 	setTempHpUseCase,
@@ -30,6 +31,8 @@ import type {
 	DomainError,
 	DomainEvent,
 	Encounter,
+	HpRange,
+	HpVariant,
 	PersistentDamageType,
 	Pf2eCreature,
 	PlayerCharacter,
@@ -59,6 +62,12 @@ type EncounterAction =
 	| { type: "edit-combatant"; id: CombatantId; newName: string }
 	| { type: "set-initiative"; id: CombatantId; value: number | undefined }
 	| { type: "set-hp"; id: CombatantId; maxHp: number | undefined }
+	| {
+			type: "set-hp-variant";
+			id: CombatantId;
+			variant: HpVariant | undefined;
+			range: HpRange;
+	  }
 	| { type: "adjust-hp"; id: CombatantId; delta: number }
 	| { type: "set-temp-hp"; id: CombatantId; tempHp: number | undefined }
 	| { type: "set-ac"; id: CombatantId; value: number | undefined }
@@ -432,6 +441,7 @@ function dispatchEncounterAction(
 		| { type: "edit-combatant" }
 		| { type: "set-initiative" }
 		| { type: "set-hp" }
+		| { type: "set-hp-variant" }
 		| { type: "adjust-hp" }
 		| { type: "set-temp-hp" }
 		| { type: "set-ac" }
@@ -472,6 +482,14 @@ function dispatchEncounterAction(
 		case "set-hp":
 			result = setHpUseCase(store, action.id, action.maxHp);
 			break;
+		case "set-hp-variant":
+			result = setHpVariantUseCase(
+				store,
+				action.id,
+				action.variant,
+				action.range,
+			);
+			break;
 		case "adjust-hp":
 			result = adjustHpUseCase(store, action.id, action.delta);
 			break;
@@ -706,6 +724,11 @@ export function useEncounter() {
 				}),
 			[],
 		),
+		setHpVariant: useCallback(
+			(id: CombatantId, variant: HpVariant | undefined, range: HpRange) =>
+				dispatch({ type: "set-hp-variant", id, variant, range }),
+			[],
+		),
 		clearEncounter: useCallback(
 			() => dispatch({ type: "clear-encounter" }),
 			[],