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>

publish.gradle

@@ -12,19 +12,17 @@ ext.configurePublishing = { packageName, packageDesc, packageVersion ->
     publishing {
         repositories {
             maven {
-                name = 'MavenCentral'
-                def releasesRepoUrl = 'https://ossrh-staging-api.central.sonatype.com/service/local/staging/deploy/maven2'
-                def snapshotsRepoUrl = 'https://central.sonatype.com/repository/maven-snapshots'
-                url = version.endsWith('SNAPSHOT') ? snapshotsRepoUrl : releasesRepoUrl
+                name = 'MrLokop'
+                url = 'https://mvn.mrlokop.dev/releases'
                 credentials {
-                    username = System.getenv('NEXUS_UPD_ID')?.trim() ?: ''
-                    password = System.getenv('NEXUS_UPD_PASS')?.trim() ?: ''
+                    username = 'AutoRobot'
+                    password = '8Mr3OAv4WcRNHZS3lAVDx32sWt71YS7GsBvN7TZhCGMmDeSz'
                 }
             }
         }
         publications {
             imgui(MavenPublication) {
-                groupId = 'io.github.spair'
+                groupId = 'org.nnstd.imgui'
                 artifactId = packageName
                 version = packageVersion