Add middleware pipeline

[rtcoder]

Summary

• Add a middleware pipeline with class-based and callable middleware support.
• Allow middleware to wrap controller dispatch, modify responses, or short-circuit requests.
• Add middleware pipeline tests for callable middleware, class middleware, and short-circuit responses.
• Document middleware usage in README and mark the roadmap item as completed.
• Replace the placeholder docs page with semantic developer documentation for the existing framework API.

Validation

• composer validate --strict --no-check-publish
• composer dump-autoload
• find Engine application tests -name '*.php' -print0 | xargs -0 -n1 php -l
• composer test
• php shift.php route:list
• php shift.php health
• HTTP smoke tests:
  • GET /health -> 200 application/json
  • GET /hello -> 404 application/json

Release

• Version label: v0.9.1