chore(psr4): phase 3 (Core hubs) — namespace Core/Module

Move Core/Module into XcVm\Core\Module (BaseModule, ModuleInterface,
MigratableInterface, ModuleLoader, ModuleManager, NavbarRegistry, NavbarItem,
CoreNavbarProvider) and Core/Module/Contract into XcVm\Core\Module\Contract
(the 6 provider interfaces). Completes the Core hub layer.

- Namespace 14 files (8 root + 6 Contract). Root files import the contracts via
  'use XcVm\Core\Module\Contract\...'; NavbarProviderInterface imports the
  root NavbarRegistry. Qualify still-global deps with leading backslash
  (\ServiceContainer, \CommandRegistry, \ModuleState, \ServerEnvironment,
  \EventDispatcher, \ListensTo, the Module exceptions, \ZipArchive,
  \InvalidArgumentException, \RuntimeException) — Container/Events/Enum/Exception
  migrate later. Migrated deps (Router, StreamPipeline, ...) keep their use.
- Rewrite the modules' 'use BaseModule/NavbarRegistry/NavbarItem;' → FQCN; add
  'use XcVm\Core\Module\...;' to other referrers across src/ and tests/.
- Fix pre-existing leading-backslash refs to the FQCN.
- Tests: qualify the module fixtures' generated 'use ModuleInterface;' /
  'use BaseModule;' / 'use NavbarRegistry;' and the registerRoutes type hint to
  FQCN; add real top-level use imports to the fixture-builder tests; update
  InterfaceContractTest registerNavbar param-type to the FQCN. Repaired
  use-inserter mis-placements in the two tests that declare a namespace() method.
- Public/index.php: class_exists('ModuleLoader') → class_exists(ModuleLoader::class).
- phpstan-baseline.neon regenerated (292→292; no new/unknown-class errors).

Verified: php -l clean; PHPStan no errors; PHPUnit 295/295; the 4 real modules
load and resolve as XcVm\Core\Module\ModuleInterface; leading-backslash
re-sweep across all migrated classes is clean.

tests/Unit/ModuleLoaderPriorityTest.php

@@ -1,5 +1,8 @@
 <?php
 
+use XcVm\Core\Module\ModuleLoader;
+use XcVm\Core\Module\NavbarRegistry;
+use XcVm\Core\Module\ModuleInterface;
 use XcVm\Core\Http\Router;
 use PHPUnit\Framework\TestCase;
 
@@ -128,11 +131,11 @@ final class ModuleLoaderPriorityTest extends TestCase {
 
         $php = '<?php' . "\n"
             . 'namespace ' . $namespace . ';' . "\n"
-            . 'use ModuleInterface;' . "\n"
+            . 'use XcVm\Core\Module\ModuleInterface;' . "\n"
             . 'use ServiceContainer;' . "\n"
             . 'use XcVm\Core\Http\Router;' . "\n"
             . 'use CommandRegistry;' . "\n"
-            . 'use NavbarRegistry;' . "\n"
+            . 'use XcVm\Core\Module\NavbarRegistry;' . "\n"
             . 'class ' . $className . ' implements ModuleInterface {' . "\n"
             . "\tpublic function getName(): string { return '{$name}'; }" . "\n"
             . "\tpublic function getVersion(): string { return '1.0.0'; }" . "\n"