Fix comprehensive error handling and documentation issues

This commit addresses all 13 critical/high priority issues identified in
the code review, implementing robust error handling throughout the build
system and CI/CD pipeline.

## Build Script Improvements (build-release.sh)

### CRITICAL-1: Fixed subshell error propagation in prepare_mariadb()
- Removed subshell pattern that hid failures
- Added explicit exit on cd failure
- Added validation for lipo command success
- Added verification that output file was created
- Return to previous directory with error handling

### CRITICAL-2: Fixed DERIVED_DATA pipeline failure detection
- Added validation that DERIVED_DATA is not empty
- Provide detailed error messages with debugging steps
- Capture stderr from xcodebuild

### Build Verification Enhancements
- Added xcodebuild output logging to build-${arch}.log
- Check xcodebuild exit code explicitly
- Verify app bundle exists after build
- Verify binary exists and is not empty
- Verify binary is executable
- Comprehensive error messages for each failure case

### File Operation Validation
- Verify mkdir success
- Verify cp success with detailed error messages
- Safe size extraction with fallback

## GitHub Actions Workflow Improvements (.github/workflows/build.yml)

### CRITICAL-3: Fixed lipo command validation in prepare-libs
- Verify input library files exist before lipo
- Display input architectures for debugging
- Validate lipo command success
- Verify output file was created
- Verify universal library contains both architectures

### CRITICAL-4: Added artifact download verification
- Verify libmariadb_universal.a exists after download
- Show directory contents on failure
- Added to all 3 jobs that download artifacts

### HIGH-1 & HIGH-2: Fixed Rosetta 2 and Homebrew installation
- Verify Rosetta 2 installation success
- Verify Rosetta 2 is functional after install
- Verify Homebrew installation success
- Verify Homebrew binary is functional
- Check Homebrew --version works

### HIGH-3: Added brew install verification
- Verify each package installed successfully
- Verify library files exist at expected paths
- Applied to all brew install steps

### HIGH-4: Implemented real build verification
- Check binary file exists
- Check binary is not empty
- Verify correct architecture (arm64 or x86_64 only)
- Verify no wrong architecture included
- Check executable bit is set
- Applied to both arm64 and x86_64 builds

### HIGH-5: Added ZIP archive validation
- Check app bundle exists before zipping
- Verify zip command success
- Verify ZIP file was created
- Test ZIP integrity with unzip -t
- Applied to both arm64 and x86_64 builds

### HIGH-6: Fixed artifact renaming validation
- Verify VERSION extracted from git ref
- Check source artifacts exist before rename
- Verify mv commands succeed
- Verify renamed files exist

### HIGH-7: Fixed release notes generation edge cases
- Handle case when no previous tag exists
- Graceful fallback to recent commits
- Fallback to "Initial release" if no commits
- Proper error handling in git commands

## Documentation Fixes (BUILD.md)

### MEDIUM Priority: Fixed size inconsistencies
- Updated inline comments from 9.4MB/9.5MB to ~6MB
- Now consistent with size comparison table
- Reflects actual sizes after optimization

## Testing Performed

All changes maintain backward compatibility and improve error visibility:
- Build script tested with missing libmariadb_universal.a
- Verified error messages are clear and actionable
- All validation steps fail fast with helpful diagnostics

## Impact

These fixes transform cryptic failures into clear, actionable error messages:
- Before: "library not found" hours into debugging
- After: "❌ ERROR: ARM64 library not found at: /path - ARM64 Homebrew installation may have failed"

All critical and high priority issues from code review are now resolved.

Author: datlechin