Change Methodology to avoid resource limits

v1.6.0: Replace shell tar with pure PHP archiving for shared hosting

Shared hosts (IONOS etc.) OOM-kill the tar process due to tight cgroup
memory limits. Replace the shell `tar czf` command with PHP PharData
which runs inside the PHP process and stays within the hosting limits.

- Use PharData to build .tar.gz archives entirely in PHP
- Add stream_upload() to S3 class for chunked multipart uploads
- Auto-detect webroot via ABSPATH (no more /var/www/html default)
- Make Object Lock headers conditional (retention_days=0 skips them)
- Reduce disk space requirement from 400MB to 100MB
- Reduce S3 multipart part size from 10MB to 5MB

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