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_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;