perf(console): case-insensitive filtering without per-line copies

Console text filtering and match highlighting lowercased a full copy of
every line, and of every drawn segment, on each pass.

- A new alloc-free ASCII case-insensitive substring search folds both
  sides on the fly, so neither the filter test nor the highlight scan
  allocates a lowercased copy per line any more.
- Both call sites use it; the case-insensitive match behaviour is
  unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui/windows/console_output_model.cpp

@@ -6,13 +6,18 @@
 namespace dragonx {
 namespace ui {
 
-namespace {
-std::string lowerCopy(std::string value)
+size_t FindCaseInsensitive(const char* hay, size_t hayLen, const std::string& needleLower, size_t from)
 {
-    std::transform(value.begin(), value.end(), value.begin(),
-                   [](unsigned char c) { return static_cast<char>(std::tolower(c)); });
-    return value;
-}
+    const size_t n = needleLower.size();
+    if (n == 0) return from <= hayLen ? from : std::string::npos;
+    if (hayLen < n) return std::string::npos;
+    auto fold = [](unsigned char c) { return (c >= 'A' && c <= 'Z') ? static_cast<unsigned char>(c + 32) : c; };
+    for (size_t i = from; i + n <= hayLen; ++i) {
+        size_t j = 0;
+        while (j < n && fold(static_cast<unsigned char>(hay[i + j])) == fold(static_cast<unsigned char>(needleLower[j]))) ++j;
+        if (j == n) return i;
+    }
+    return std::string::npos;
 }
 
 bool consoleLinePassesFilter(const std::string& lineText,
@@ -25,9 +30,7 @@ bool consoleLinePassesFilter(const std::string& lineText,
     if (!filter.appMessagesEnabled && channel == ConsoleChannel::App) return false;
     if (filter.errorsOnly && channel != ConsoleChannel::Error) return false;
     if (!filter.text.empty()) {
-        std::string needle = lowerCopy(filter.text);
-        std::string haystack = lowerCopy(lineText);
-        if (haystack.find(needle) == std::string::npos) return false;
+        if (FindCaseInsensitive(lineText.data(), lineText.size(), filter.text, 0) == std::string::npos) return false;
     }
     return true;
 }

src/ui/windows/console_output_model.h

@@ -18,6 +18,10 @@ struct ConsoleOutputFilter {
     bool appMessagesEnabled = true;      // App channel
 };
 
+// Alloc-free ASCII case-insensitive substring search (C-locale fold). Returns the byte
+// offset of the first match at or after `from`, or npos. Needle is folded on the fly.
+size_t FindCaseInsensitive(const char* hay, size_t hayLen, const std::string& needle, size_t from);
+
 // Whether a line with the given channel + text passes the active filter.
 bool consoleLinePassesFilter(const std::string& lineText,
                              ConsoleChannel channel,

src/ui/windows/console_tab.cpp

@@ -1196,11 +1196,9 @@ void ConsoleTab::drawVisibleLines(float padX, float lineHeight, bool hasTextFilt
 
             // Filter match highlight (case-insensitive) within this segment.
             if (hasTextFilter && !filterLower.empty() && seg.byteStart < seg.byteEnd) {
-                std::string segLower(segStart, segEnd);
-                std::transform(segLower.begin(), segLower.end(), segLower.begin(),
-                               [](unsigned char c) { return static_cast<char>(std::tolower(c)); });
+                const size_t segBytes = static_cast<size_t>(segEnd - segStart);
                 size_t pos = 0;
-                while ((pos = segLower.find(filterLower, pos)) != std::string::npos) {
+                while ((pos = FindCaseInsensitive(segStart, segBytes, filterLower, pos)) != std::string::npos) {
                     float xs = font->CalcTextSizeA(fontSize, FLT_MAX, 0, segStart, segStart + static_cast<int>(pos)).x;
                     float xe = font->CalcTextSizeA(fontSize, FLT_MAX, 0, segStart,
                                                    segStart + static_cast<int>(pos + filterLower.size())).x;