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/controller.h

@@ -148,6 +148,7 @@ private:
     std::unique_ptr<SnapshotProvider> m_snapshotProv;
     PageMap         m_prevPages;
     QSet<int64_t>   m_changedOffsets;
+    QHash<uint64_t, ValueHistory> m_valueHistory;
     uint64_t        m_refreshGen = 0;
     uint64_t        m_readGen = 0;
     bool            m_readInFlight = false;