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>

packages/domain/src/__tests__/hp-range.test.ts

@@ -0,0 +1,78 @@
+import { describe, expect, it } from "vitest";
+import { hpForVariant, hpRange } from "../hp-range.js";
+
+describe("hpRange", () => {
+	it("derives min and max from a formula with a positive modifier", () => {
+		const range = hpRange({ average: 16, formula: "3d6 + 6" });
+		expect(range).toEqual({ min: 9, average: 16, max: 24 });
+	});
+
+	it("derives min and max from a formula without a modifier", () => {
+		const range = hpRange({ average: 40, formula: "9d8" });
+		expect(range).toEqual({ min: 9, average: 40, max: 72 });
+	});
+
+	it("subtracts a negative modifier", () => {
+		const range = hpRange({ average: 45, formula: "10d8 - 5" });
+		expect(range).toEqual({ min: 5, average: 45, max: 75 });
+	});
+
+	it("treats an en dash as a minus sign", () => {
+		// Bundled bestiary data contains e.g. "2d6 – 2" (U+2013).
+		const range = hpRange({ average: 5, formula: "2d6 – 2" });
+		expect(range).toEqual({ min: 1, average: 5, max: 10 });
+	});
+
+	it("never drops below 1 HP when the modifier exceeds the dice", () => {
+		const range = hpRange({ average: 1, formula: "1d4 - 10" });
+		expect(range?.min).toBe(1);
+		expect(range?.max).toBe(1);
+	});
+
+	it("handles large pools without whitespace", () => {
+		expect(hpRange({ average: 256, formula: "19d12+133" })).toEqual({
+			min: 152,
+			average: 256,
+			max: 361,
+		});
+	});
+
+	it("returns null for a missing formula", () => {
+		expect(hpRange({ average: 7, formula: "" })).toBeNull();
+	});
+
+	it("returns null for prose instead of a dice pool", () => {
+		expect(
+			hpRange({ average: 0, formula: "equal to the summoner's" }),
+		).toBeNull();
+	});
+
+	it("returns null for a flat number", () => {
+		expect(hpRange({ average: 50, formula: "50" })).toBeNull();
+	});
+
+	it("returns null for a pool with zero dice or zero sides", () => {
+		expect(hpRange({ average: 0, formula: "0d6" })).toBeNull();
+		expect(hpRange({ average: 0, formula: "2d0" })).toBeNull();
+	});
+
+	it("returns null for compound formulas it cannot reason about", () => {
+		expect(hpRange({ average: 20, formula: "2d6 + 2d8" })).toBeNull();
+	});
+});
+
+describe("hpForVariant", () => {
+	const range = { min: 9, average: 16, max: 24 };
+
+	it("returns the average when no variant is set", () => {
+		expect(hpForVariant(range, undefined)).toBe(16);
+	});
+
+	it("returns the minimum for the min variant", () => {
+		expect(hpForVariant(range, "min")).toBe(9);
+	});
+
+	it("returns the maximum for the max variant", () => {
+		expect(hpForVariant(range, "max")).toBe(24);
+	});
+});