add analysis fixes, PHP 8.x fixes, create hello world pages

[Bryn199]

Summary

Type of change

• Phase A (setup, analysis docs)
• Phase B (runtime compatibility fix)
• Phase C (dependency upgrade)
• Phase D (test / CI improvement)
• Phase E (AI-native feature, rebirth)
• Documentation only
• Bugfix (out of phase work)
• Other:

Related issue

What changed

• Replaced Travis CI with GitHub Actions (PHP 8.1 / 8.2 / 8.3)
• Added Docker setup for local development
• Updated composer.json for PHP 8.1+ compatibility
• Fixed 5 PHP 8.x breakages in fuel/core
• Added Hello World page at /helloworld as a smoke test
• Added ARCHITECTURE.md to document the project structure

Why

FuelPHP 1.8 was written for PHP 5.4 and breaks on modern PHP. This PR gets the framework booting cleanly on PHP 8.x and gives developers a working local setup from day one.

How to test

composer install --no-scripts
php -S localhost:8000 public/index.php
# Open http://localhost:8000/helloworld
# Expected: Hello World! — Welcome to LeFuel.

Checklist

• CI is green
• Tests added or updated when applicable
• Documentation updated when applicable
• PR is small and focused (one concern)
• I have read CONTRIBUTING.md
• I have not committed AI-generated code I cannot explain
• I have not committed credentials, secrets, or personal data

Screenshots / demo