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/warm.json

@@ -22,6 +22,9 @@
     "indHoverSpan": "#AA9565",
     "indCmdPill": "#2a2a2a",
     "indDataChanged": "#6B959F",
+    "indHeatCold": "#6B959F",
+    "indHeatWarm": "#AA9565",
+    "indHeatHot": "#A05040",
     "indHintGreen": "#464646",
     "markerPtr": "#6B3B21",
     "markerCycle": "#AA9565",