HP formula stored on participant, loads in edit modal

hpFormula field persisted on participant doc (makeParticipant + builder).
Add handler stores formula. Edit modal loads participant.hpFormula, reroll
button sets maxHp field (no save until save). Edit submit persists formula.
Formula only for monster/npc types.

shared/tests/turn.hpformula.test.js

@@ -0,0 +1,60 @@
+// HP formula roll: parse "2d6+9" style, roll, return total.
+const shared = require('@ttrpg/shared');
+const { rollHpFormula } = shared;
+
+describe('rollHpFormula', () => {
+  test('basic NdM', () => {
+    const r = rollHpFormula('2d6');
+    expect(r).toBeGreaterThanOrEqual(2);
+    expect(r).toBeLessThanOrEqual(12);
+  });
+
+  test('NdM+bonus', () => {
+    const r = rollHpFormula('2d6+9');
+    expect(r).toBeGreaterThanOrEqual(11);
+    expect(r).toBeLessThanOrEqual(21);
+  });
+
+  test('NdM-bonus', () => {
+    const r = rollHpFormula('3d8-2');
+    expect(r).toBeGreaterThanOrEqual(1);
+    expect(r).toBeLessThanOrEqual(22);
+  });
+
+  test('single die', () => {
+    const r = rollHpFormula('1d20');
+    expect(r).toBeGreaterThanOrEqual(1);
+    expect(r).toBeLessThanOrEqual(20);
+  });
+
+  test('case insensitive', () => {
+    const r = rollHpFormula('2D6+5');
+    expect(r).toBeGreaterThanOrEqual(7);
+    expect(r).toBeLessThanOrEqual(17);
+  });
+
+  test('whitespace tolerant', () => {
+    const r = rollHpFormula(' 2d6 + 9 ');
+    expect(r).toBeGreaterThanOrEqual(11);
+    expect(r).toBeLessThanOrEqual(21);
+  });
+
+  test('large die', () => {
+    const r = rollHpFormula('4d12+10');
+    expect(r).toBeGreaterThanOrEqual(14);
+    expect(r).toBeLessThanOrEqual(58);
+  });
+
+  test('invalid returns null', () => {
+    expect(rollHpFormula('abc')).toBeNull();
+    expect(rollHpFormula('')).toBeNull();
+    expect(rollHpFormula(null)).toBeNull();
+    expect(rollHpFormula(undefined)).toBeNull();
+    expect(rollHpFormula('2d')).toBeNull();
+    expect(rollHpFormula('d6')).toBeNull();
+  });
+
+  test('zero dice invalid', () => {
+    expect(rollHpFormula('0d6')).toBeNull();
+  });
+});