perf: stop per-frame SQLite writes and console spam, make perf.log opt-in

- Explorer: the tip check runs every frame the Explorer tab is open,
  and it read both tip rows and rewrote them each time (two autocommit
  WAL writes per frame). A steady tip is now kept in memory and skips
  SQLite. A new tip writes both rows in one savepoint, so a crash
  between them can't pair a new height with the old hash. A test
  checks that 100 steady checks write nothing.
- Console: while the daemon was in the Error state the Console tab
  added its error line on every poll, i.e. every frame, re-laying out
  the whole buffer each time. It prints once per error now: on entering
  Error, when the message changes, or after another crash
  (EmbeddedDaemon::getUnexpectedExitCount(), never reset). setState
  writes the message before the state, so a reader that sees Error sees
  that error's message. The stray " ===" suffix is gone.
- perf.log: the profiler appended a block every 300 frames on every
  run and never rotated the file (tens of MB a day for a wallet left
  open). It is now off unless the app starts with DRAGONX_PERFLOG=1;
  otherwise its zone hooks are near no-ops.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/explorer/explorer_block_cache.h

@@ -56,11 +56,15 @@ public:
                                  const std::string& currentBestHash);
     void updateTip(int height, const std::string& hash);
 
+    // Rows changed through this connection since it opened (sqlite3_total_changes); 0 when closed.
+    // Lets tests check that a steady tip writes nothing.
+    int totalChanges() const;
+
 private:
     bool exec(const char* sql);
     std::string getMetaValue(const std::string& key);
     int getMetaInt(const std::string& key, int fallback);
-    void setMetaValue(const std::string& key, const std::string& value);
+    bool setMetaValue(const std::string& key, const std::string& value);
     bool createSchema();
     void migrateLegacyJsonIfNeeded();
     void close();
@@ -68,6 +72,12 @@ private:
     sqlite3* db_ = nullptr;
     std::string database_path_;
     std::string legacy_json_path_;
+
+    // The saved tip as last written or read, so the per-frame check can skip SQLite while the chain
+    // tip hasn't moved. The database stays the source of truth; this is only a copy of it.
+    bool tip_known_ = false;
+    int tip_height_ = 0;
+    std::string tip_hash_;
 };
 
 } // namespace ui