[adrianhall]

Summary

OfflineDbContext.BuildDatasyncOfflineOptions()      (called by PullAsync, and by push via Lazy<OfflineOptions>)
- DisposableLock.AcquireLock()                      -> SemaphoreSlim.Wait()   [UnsupportedOSPlatform("browser")]
+ lock (Lock _optionsLock)                          -> Monitor, fine on single-threaded wasm
    build OfflineOptions once

src/.../Threading/
 ├── DisposableLock.cs         - AcquireLock()          (sync, now unused)
 └── AsyncLockDictionary.cs    - AcquireLock(string)    (sync, now unused)

• Fixes BuildDatasyncOfflineOptions throws PlatformNotSupportedException on .NET 11 Blazor WebAssembly #574: .NET 11 enforces [UnsupportedOSPlatform("browser")] on blocking SemaphoreSlim.Wait(), so the first PullAsync threw PlatformNotSupportedException on browser-wasm. Based on the approach in Avoid blocking SemaphoreSlim.Wait in BuildDatasyncOfflineOptions for browser-wasm #575 (thanks @AtakanUk).
• Fixes Remove unused synchronous AcquireLock methods (DisposableLock, AsyncLockDictionary) #597: with the above, nothing in src/ calls the synchronous AcquireLock methods. Removed them and their tests; the async variants stay.
• Docs: blazor-wasm.md now lists the two remaining known browser-wasm limitations (sync SaveChanges() and MD5 query IDs) rather than leaving them undiscovered. Blazor WASM offline mode remains unsupported.

Evidence

• Before: BuildDatasyncOfflineOptions() reaches SemaphoreSlim.Wait() (CA1416 flags it with SupportedPlatform=browser; throws on .NET 11 browser-wasm per BuildDatasyncOfflineOptions throws PlatformNotSupportedException on .NET 11 Blazor WebAssembly #574).
  After: no blocking semaphore wait remains in the library.
• New tests:

  BuildDatasyncOfflineOptions_ReturnsSameInstance:   two calls -> same instance, configurator ran once
  BuildDatasyncOfflineOptions_ConcurrentCalls_BuildOnce: 16 parallel calls -> same instance, configurator ran once
  

• Full solution dotnet test: all 12 test projects pass (Client: 1445 passed, 0 failed).
• Not verified on an actual .NET 11 browser-wasm runtime; the repo targets net10.0.

Merge Danger

Door: two-way

Blast Radius: internal client locking

• _optionsLock and both removed methods are internal; no public API change.
• Behavioral differences from lock vs SemaphoreSlim: re-entrancy is now allowed (a user's OnDatasyncInitialization that re-enters BuildDatasyncOfflineOptions would recurse instead of deadlock, both are user bugs), and calling it on a disposed context no longer throws from the semaphore.
• Not addressed here (documented on BuildDatasyncOfflineOptions throws PlatformNotSupportedException on .NET 11 Blazor WebAssembly #574): MD5.HashData in PullRequestBuilder and the sync-over-async in UpdateOperationsQueue().