Refactor, tests

Improvements

• Add PhpUnit tests with max coverage.
• Apply code scanners: PHP syntax checking, phpcs (standards, PHP compatibility), phpstan
• Refactor Lando testing env
• More sophisticated debug code generation
• Strict type checking
• Turn code into classes for better testing and architecture
• Fix handful of small bugs (revealed by tests)
• Commandline php _ping.php defaults to debug mode, and reveals the debug token.
• No functional changes.