refactor: move PSR-15 classes inside the Http boundary

[tyrsson]

Closes #17.

Relocates the PSR-15 classes under the Http\ perimeter so the ecosystem guard rule
(Psr\Http\Server\** usable only from Webware\**\Http\**) passes once webware-tools
0.1.x is updated. This is what unblocks #18.

• src/Middleware/* -> src/Http/Middleware/*.
• MessageMiddlewareFactory -> Http\Middleware\Container\. The new
  Http\**\Middleware\* structural rule requires every class directly in that
  namespace to be named *Middleware and implement PSR-15 MiddlewareInterface,
  so the factory cannot stay co-located; it nests in Container\, matching
  webware-acl, webware-usermanager and webware-mailer.
• MissingSessionException::forMiddleware() now takes the middleware class name
  (string $middlewareClass, documented as class-string) instead of a PSR-15
  instance, which drops the exception's Psr\Http\Server dependency.
• Unit tests mirror the new src layout; README and docs/v1/* wiring examples updated.

Verified against webware-tools 0.1.x (mago 1.48.1, guard.mode = "default" with the
perimeter rules active — swapped into the ignored vendor copy for the run, so
composer.lock is untouched): mago fmt --check, mago lint, mago analyze and
mago guard are all clean, and phpunit passes 70 unit + 1 integration test.
Commit is signed off.

Merge up: lands on 1.0.x first.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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