docs(building): update BUILDING.md with iOS prerequisites and clean-build script

Updates BUILDING.md to reflect recent changes in build-native.sh, especially
the Xcode Command Line Tools prerequisite check and the clean-build script.

Problem:
- BUILDING.md didn't mention Xcode Command Line Tools prerequisite
  (recently added to build-native.sh)
- clean-build.sh script exists but wasn't documented
- iOS build troubleshooting lacked Command Line Tools guidance

Changes:
- Add Xcode Command Line Tools to Prerequisites section
  - Document installation command (xcode-select --install)
  - Include verification steps (xcode-select -p, xcodebuild -version)
  - Note that build script automatically checks for these tools
  - Explain that sqlite3 is part of Command Line Tools

- Document clean-build.sh script in Build Scripts section
  - Basic usage: ./scripts/clean-build.sh
  - All options: --all, --clean-gradle-cache, --clean-derived-data,
    --reinstall-node
  - Explain when to use clean builds

- Enhance iOS Native Build Process section
  - Add prerequisite note about Command Line Tools
  - Include troubleshooting commands for pod install issues
  - Reference prerequisites section for details

- Add comprehensive troubleshooting sections
  - Clean Build section at start of Troubleshooting
    - Recommends clean-build as first step for many issues
    - Lists when to use clean builds
  - iOS Build Issues section
    - Command Line Tools configuration errors
    - SQLite/linker issues and pkgx conflicts
    - CocoaPods installation problems
    - All with clear solutions and commands

The documentation now accurately reflects:
- Xcode Command Line Tools as required iOS prerequisite
- clean-build.sh as available build tool
- Complete iOS troubleshooting workflow

Files modified:
- BUILDING.md
