Documents the whole framework in docs/framework.md, including a
worked tutorial. Building and verifying that tutorial end-to-end
surfaced three real bugs, now fixed:

- MigrateCommand re-ran every migration file on every invocation
  instead of skipping ones already logged in the migrations table,
  so a second `migrate` always failed once tables existed.
- Router's 404/400/500 handlers sent a 200 OK regardless of the
  error, since the returned string was never given the matching
  status code.
- Several views (nav login state, product order errors) and
  OrderProductController read/wrote the raw $_SESSION superglobal
  directly, bypassing the session driver's key prefix, so they
  silently never saw values written via session()->put().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>