NumberField draft buffer; surface massive-damage / death-at-0 rules in combat

- NumberField keeps a focus-time string draft so min-bounded fields (point-buy, HP,
  quantities) no longer snap to the minimum on every keystroke while retyping; the
  committed value is still always a clamped integer.
- Combat tracker now logs the 5e death rules when a downed PC takes damage: instant
  death from massive damage (leftover >= max HP), otherwise a reminder to mark a death
  save failure (two on a crit). Surfaced, not auto-applied — the player owns their
  character's death-save count (consistent with the no-auto-roll design). New tested
  isMassiveDamageDeath() engine helper.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/combat/engine.test.ts

@@ -7,6 +7,7 @@ import {
   applyInitiatives,
   currentCombatant,
   endEncounter,
+  isMassiveDamageDeath,
   moveCombatant,
   nextTurn,
   previousTurn,
@@ -216,3 +217,13 @@ describe('applyInitiatives', () => {
     expect(currentCombatant(e)?.id).toBe('b');
   });
 });
+
+describe('isMassiveDamageDeath', () => {
+  it('kills when leftover damage past 0 is >= max HP', () => {
+    // 12-max PC at 4 HP takes 17 → current -13; 13 >= 12 → instant death
+    expect(isMassiveDamageDeath(12, -13)).toBe(true);
+    expect(isMassiveDamageDeath(12, -12)).toBe(true); // exactly max → dead
+    expect(isMassiveDamageDeath(12, -11)).toBe(false); // one short → dying, not dead
+    expect(isMassiveDamageDeath(12, 0)).toBe(false); // dropped to 0, no overflow
+  });
+});