feat: add IMGUI_USE_WCHAR32 support, cross-compile natives, publish to mvn.mrlokop.dev

- Enable IMGUI_USE_WCHAR32 in imconfig.h (ImWchar = uint32_t)
- Extend ImFontGlyphRangesBuilder, ImFontConfig, ImFontAtlas for 32-bit codepoints
- Cross-compile natives for macOS universal, Linux x86_64 (glibc), Windows x86_64
- Publish as org.nnstd.imgui:...:1.90.0-wchar32+mrlokop2 to mvn.mrlokop.dev
- Add BUILDING.md with full rebuild and publish instructions

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

buildSrc/src/main/groovy/tool/generator/GenerateLibs.groovy

@@ -108,12 +108,14 @@ class GenerateLibs extends DefaultTask {
 
         if (forWindows) {
             def win64 = BuildTarget.newDefaultTarget(Os.Windows, Architecture.Bitness._64)
+            win64.compilerPrefix = 'x86_64-w64-mingw32-'
             addFreeTypeIfEnabled(win64)
             buildTargets += win64
         }
 
         if (forLinux) {
             def linux64 = BuildTarget.newDefaultTarget(Os.Linux, Architecture.Bitness._64)
+            linux64.compilerPrefix = 'x86_64-unknown-linux-gnu-'
             addFreeTypeIfEnabled(linux64)
             buildTargets += linux64
         }