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/components/ui/NumberField.tsx

@@ -1,6 +1,12 @@
+import { useState } from 'react';
 import { cn } from '@/lib/cn';
 
-/** Integer input that never emits NaN — coerces blanks/garbage to a fallback. */
+/**
+ * Integer input that never emits NaN. While focused it keeps a local string draft
+ * so you can transiently clear or partially type a value (e.g. retype a min-bounded
+ * field) without it snapping to the minimum on every keystroke. The committed value
+ * is always a finite, clamped integer.
+ */
 export function NumberField({
   value,
   onChange,
@@ -16,21 +22,43 @@ export function NumberField({
   className?: string;
   'aria-label'?: string;
 }) {
+  const [draft, setDraft] = useState<string | null>(null);
+  const clamp = (n: number) => {
+    let v = Math.trunc(n);
+    if (min !== undefined) v = Math.max(min, v);
+    if (max !== undefined) v = Math.min(max, v);
+    return v;
+  };
+  // While focused (draft !== null) show the raw text; otherwise the controlled value.
+  const display = draft !== null ? draft : Number.isFinite(value) ? String(value) : '0';
   return (
     <input
       type="number"
       inputMode="numeric"
       aria-label={ariaLabel}
-      value={Number.isFinite(value) ? value : 0}
+      value={display}
       min={min}
       max={max}
+      onFocus={(e) => setDraft(e.target.value)}
       onChange={(e) => {
-        const raw = Number(e.target.value);
-        let n = Number.isFinite(raw) ? Math.trunc(raw) : 0;
-        if (min !== undefined) n = Math.max(min, n);
-        if (max !== undefined) n = Math.min(max, n);
-        onChange(n);
+        const raw = e.target.value;
+        setDraft(raw);
+        // Emit a clamped value live (so previews update) only when the text parses;
+        // an empty/"-" intermediate leaves the committed value untouched.
+        if (raw.trim() !== '' && raw !== '-') {
+          const n = Number(raw);
+          if (Number.isFinite(n)) onChange(clamp(n));
+        }
       }}
+      onBlur={(e) => {
+        const raw = e.target.value;
+        if (raw.trim() !== '' && raw !== '-') {
+          const n = Number(raw);
+          if (Number.isFinite(n)) onChange(clamp(n));
+        }
+        setDraft(null); // resume showing the controlled value
+      }}
+      onKeyDown={(e) => { if (e.key === 'Enter') (e.target as HTMLInputElement).blur(); }}
       className={cn(
         'w-full rounded-md border border-line bg-surface px-2 py-1.5 text-center text-sm text-ink',
         'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent/60',