Registry lockdown phase after initialization

[willgriffin]

Problem

The ObjectRegistry can be mutated at any time — during startup, during request handling, during getInheritanceChain(), etc. This makes it impossible to reason about the registry state at any given point and prevents caching optimizations.

Proposed Solution

Introduce a RegistryLockdown phase:

1. After all packages are loaded and classes registered, call ObjectRegistry.lock()
2. After lockdown:
   • register() throws ConfigurationError("Registry is locked")
   • registerFromManifest() throws similarly
   • All caches are frozen (inheritance chains, field maps, schemas)
   • Read operations guaranteed stable
3. Provide ObjectRegistry.isLocked() for conditional checks
4. In test environments, provide ObjectRegistry.unlock() for test setup/teardown

Integration Points

• CLI: Lock after manifest discovery completes
• SvelteKit hooks: Lock after withBackgroundJobs() / server initialization
• Vitest plugin: Lock after test manifest generation, unlock in afterAll

Key Files

• packages/core/src/registry.ts

Part of epic #1003.