feat: value history heatmap, write-fail guard, crash handler hardening

- Value history ring buffer (10 slots) tracks per-node change frequency
- Three-level heatmap: cold (blue), warm (amber), hot (red) via theme
- Heat persists indefinitely (no fade) — shows analysis history
- Calltip on hover shows previous values list
- Old themes auto-derive heat colors from existing palette
- Write failures no longer apply optimistic visual updates
- Crash handler: re-entrancy guard, context dump before risky APIs

src/themes/theme.cpp

@@ -28,6 +28,9 @@ const ThemeFieldMeta kThemeFields[] = {
     {"indHoverSpan",  "Hover Span",     "Indicators",  &Theme::indHoverSpan},
     {"indCmdPill",    "Cmd Pill",       "Indicators",  &Theme::indCmdPill},
     {"indDataChanged","Data Changed",   "Indicators",  &Theme::indDataChanged},
+    {"indHeatCold",   "Heat Cold",      "Indicators",  &Theme::indHeatCold},
+    {"indHeatWarm",   "Heat Warm",      "Indicators",  &Theme::indHeatWarm},
+    {"indHeatHot",    "Heat Hot",       "Indicators",  &Theme::indHeatHot},
     {"indHintGreen",  "Hint Green",     "Indicators",  &Theme::indHintGreen},
     {"markerPtr",     "Pointer",        "Markers",     &Theme::markerPtr},
     {"markerCycle",   "Cycle",          "Markers",     &Theme::markerCycle},
@@ -50,6 +53,14 @@ Theme Theme::fromJson(const QJsonObject& o) {
         if (o.contains(kThemeFields[i].key))
             t.*kThemeFields[i].ptr = QColor(o[kThemeFields[i].key].toString());
     }
+    // Derive heat colors from the theme's own palette when keys are absent
+    // cold = keyword blue, warm = hover/string amber, hot = marker red
+    if (!t.indHeatCold.isValid())
+        t.indHeatCold = t.syntaxKeyword;
+    if (!t.indHeatWarm.isValid())
+        t.indHeatWarm = t.indHoverSpan.isValid() ? t.indHoverSpan : t.syntaxString;
+    if (!t.indHeatHot.isValid())
+        t.indHeatHot = t.markerPtr;
     return t;
 }