RegMeta::ApplyEditAndContinue mutates state without obtaining locks

`RegMeta::` mutates its `CMiniMdRW` through `ApplyDelta` without acquiring the `RegMeta` read/write semaphore.

Other exported metadata APIs acquire that semaphore when `MetaDataThreadSafetyOptions` is enabled. A concurrent reader or writer can therefore access metadata while `ApplyDelta` changes tables, heaps, and lookup caches.

## Reachability

The path is exposed through the native metadata dispenser:

1. Obtain `IMetaDataDispenserEx`.
2. Enable `MetaDataThreadSafetyOptions`.
3. Open a writable scope and request `IMetaDataEmit`.
4. Call `IMetaDataEmit::ApplyEditAndContinue`.
5. Concurrently call another metadata API on the same scope.

This is not the `MetadataUpdater.ApplyUpdate` path used in HR scenarios, which uses the separately locked `MDInternalRW::ApplyEditAndContinue`.

Begin at IMetaDataEmit::ApplyEditAndContinue and trace RegMeta::ApplyEditAndContinue through CMiniMdRW::ApplyDelta, comparing the locking behavior of other exported metadata APIs and MDInternalRW::ApplyEditAndContinue. Reproduce the concurrent reader/writer scenario with thread safety enabled; done when ApplyEditAndContinue is protected by the same semaphore and concurrent access is safe.

Written by the indexing model from the issue text.