chore: clean up generated mobile artifacts

[pwltr]

Summary

• add split iOS, Android, and lightweight bindgen validation workflows
• stop tracking generated Android Kotlin bindings and iOS XCFramework release payloads
• generate iOS XCFramework artifacts under dist/ios with deterministic zip metadata
• document and ignore generated mobile binary outputs

See #114

Validation

• ruby -e "require 'yaml'; Dir['.github/workflows/*.{yml,yaml}'].sort.each { |f| YAML.load_file(f); puts "ok #{f}" }"
• bash -n build.sh build_ios.sh build_android.sh build_python.sh
• git diff --check
• cargo metadata --locked --format-version 1
• swift package dump-package
• ./gradlew projects --no-daemon