feat: value history timestamps, Ctrl+F search, base address fixes

- Add timestamps to ValueHistory ring buffer, expose via new MCP tool
  node.history, show relative time in popup ("26s ago", "2m ago")
- Add "Clear Value History" right-click menu for single and multi-select
- Add Ctrl+F find bar to RcxEditor with live search, Enter-to-next, wrap
- Fix Ctrl+F in workspace dock to auto-focus search field
- Add "Change to float" quick-convert for Hex32 right-click menu
- Sort workspace explorer by children count descending (most fields first)
- Fix provider->base() overwriting saved base address from .rcx files
- Add formula support to MCP change_base operation
- Re-evaluate baseAddressFormula on provider attach in selectSource()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core.h

@@ -11,6 +11,7 @@
 #include <array>
 #include <memory>
 #include <variant>
+#include <QDateTime>
 
 #include "providers/provider.h"
 #include "providers/buffer_provider.h"
@@ -500,6 +501,7 @@ struct NodeTree {
 struct ValueHistory {
     static constexpr int kCapacity = 10;
     std::array<QString, kCapacity> values;
+    std::array<qint64, kCapacity> timestamps{};  // msec since epoch
     int count = 0;   // total unique values recorded
     int head  = 0;   // next write position in ring
 
@@ -509,10 +511,16 @@ struct ValueHistory {
             if (values[last] == v) return;  // no change
         }
         values[head] = v;
+        timestamps[head] = QDateTime::currentMSecsSinceEpoch();
         head = (head + 1) % kCapacity;
         if (count < INT_MAX) count++;
     }
 
+    void clear() {
+        count = 0;
+        head = 0;
+    }
+
     int uniqueCount() const { return qMin(count, kCapacity); }
 
     // 0=static, 1=cold(2 unique), 2=warm(3-4), 3=hot(5+)
@@ -536,6 +544,17 @@ struct ValueHistory {
         for (int i = 0; i < n; i++)
             fn(values[(start + i) % kCapacity]);
     }
+
+    // Iterate with timestamps from oldest to newest
+    template<typename Fn>
+    void forEachWithTime(Fn&& fn) const {
+        int n = uniqueCount();
+        int start = (head + kCapacity - n) % kCapacity;
+        for (int i = 0; i < n; i++) {
+            int idx = (start + i) % kCapacity;
+            fn(values[idx], timestamps[idx]);
+        }
+    }
 };
 
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