Optimize logger string formatting to avoid repeated allocations (#670)

Addresses feedback on PR #668 to eliminate inefficient string operations
in the logger's placeholder replacement logic.

### Changes

- **Removed repeated `find()` and `replace()` calls** that allocated new
buffers on each iteration
- **Build output directly into stringstream** by iterating through
`header_` once, appending arguments when "%@" placeholders are
encountered

### Before
```cpp
std::string formattedHeader = header_;
while ((pos = formattedHeader.find("%@", pos)) != std::string::npos) {
  formattedHeader.replace(pos, 2, argStrings[argIndex]);  // allocates + copies
  pos += argStrings[argIndex].length();
  ++argIndex;
}
ss << formattedHeader;
```

### After
```cpp
for (size_t i = 0; i < header_.size(); ++i) {
  if (i + 1 < header_.size() && header_[i] == '%' && header_[i + 1] == '@' &&
      argIndex < argStrings.size()) {
    ss << argStrings[argIndex];
    ++argIndex;
    ++i;
  } else {
    ss << header_[i];
  }
}
```

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: chhwang <[email protected]>

Author: Copilot

src/include/logger.hpp

@@ -92,19 +92,20 @@ class Logger {
     std::array<std::string, sizeof...(args)> argStrings = {toStringHelper(std::forward<Args>(args))...};
 
     std::stringstream ss;
-    std::string formattedHeader = header_;
     size_t argIndex = 0;
-    size_t pos = 0;
 
-    // Replace "%@" placeholders
-    while ((pos = formattedHeader.find("%@", pos)) != std::string::npos && argIndex < argStrings.size()) {
-      formattedHeader.replace(pos, 2, argStrings[argIndex]);
-      pos += argStrings[argIndex].length();
-      ++argIndex;
+    // Replace "%@" placeholders by iterating through header_
+    for (size_t i = 0; i < header_.size(); ++i) {
+      if (i + 1 < header_.size() && header_[i] == '%' && header_[i + 1] == '@' &&
+          argIndex < argStrings.size()) {
+        ss << argStrings[argIndex];
+        ++argIndex;
+        ++i;  // Skip the '@' character
+      } else {
+        ss << header_[i];
+      }
     }
 
-    ss << formattedHeader;
-
     // Append remaining arguments
     for (size_t i = argIndex; i < argStrings.size(); ++i) {
       if (delimiter_) {