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/defaults/vs.json

@@ -22,6 +22,9 @@
     "indHoverSpan": "#b180d7",
     "indCmdPill": "#2d2d30",
     "indDataChanged": "#8fbc7a",
+    "indHeatCold": "#569cd6",
+    "indHeatWarm": "#d69d85",
+    "indHeatHot": "#f44747",
     "indHintGreen": "#5a8248",
     "markerPtr": "#f44747",
     "markerCycle": "#e5a00d",