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);
+	});
+});