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/ModuleStateEnumTest.php

@@ -1,5 +1,6 @@
 <?php
 
+use XcVm\Core\Enum\ModuleState;
 use XcVm\Core\Module\ModuleManager;
 use PHPUnit\Framework\TestCase;