chore(psr4): phase 3 (Core) — namespace Core/Enum + Core/Exception

- Core/Enum → XcVm\Core\Enum (BootContext, ModuleState, ServerEnvironment).
- Core/Exception → XcVm\Core\Exception (XcVmException) + \Container
  (ContainerException, CircularDependencyException, ServiceCreationException) +
  \Module (ModuleException + the 4 module exceptions). The hierarchy resolves:
  XcVmException extends \RuntimeException; the Container/Module exceptions extend
  XcVmException via use; ContainerException keeps its PSR ContainerExceptionInterface.
- Add use to referrers across src/ and tests/; fix the leading-backslash refs
  qualified in earlier hub commits (\ModuleState, \ServerEnvironment,
  \XcVmException, \ContainerException, \ModuleLoadException, ...) to their FQCNs.
- phpstan-baseline.neon regenerated (292→292).

Verified: php -l clean; PHPStan no errors (first pass); PHPUnit 295/295;
leading-backslash re-sweep clean.

tests/Unit/ExceptionHierarchyTest.php

@@ -1,6 +1,15 @@
 <?php
 
 declare(strict_types=1);
+use XcVm\Core\Exception\Module\ModuleNotFoundException;
+use XcVm\Core\Exception\Module\ModuleManifestException;
+use XcVm\Core\Exception\Module\ModuleLoadException;
+use XcVm\Core\Exception\Module\ModuleCycleException;
+use XcVm\Core\Exception\Module\ModuleException;
+use XcVm\Core\Exception\Container\ServiceCreationException;
+use XcVm\Core\Exception\Container\CircularDependencyException;
+use XcVm\Core\Exception\Container\ContainerException;
+use XcVm\Core\Exception\XcVmException;
 use XcVm\Core\Container\Psr\NotFoundException;
 use XcVm\Core\Container\Psr\NotFoundExceptionInterface;
 use XcVm\Core\Container\Psr\ContainerExceptionInterface;