Feat: WebGL, extract Threading/Task APIs, gate unsupported systems, and align async model

[NickKhalow]

WebGL runs in a single-threaded environment and does not support System.Threading, System.Threading.Tasks, blocking synchronization primitives, file system access, or native sockets.
This PR removes or gates incompatible APIs, replaces them with WebGL-safe abstractions, and enforces compile-time correctness to prevent regressions.

Summary of Changes

Threading & Synchronization

• Removed direct usage of System.Threading primitives (SemaphoreSlim, Interlocked, thread pool usage).
• Introduced WebGL-safe wrappers (DCLSemaphoreSlim, DCLInterlocked, DCLTask).
• MultiThreadSync and ECS locking systems are excluded from WebGL builds (#if !UNITY_WEBGL), as WebGL execution is single-threaded by design.
• Static analysis markers added to explicitly acknowledge WebGL thread-safety assumptions where unavoidable.

Task to UniTask migration

• Eliminated System.Threading.Tasks usage across runtime and infrastructure code.
• Replaced Task / ValueTask APIs with UniTask.
• GLTFast integrations are fully excluded from WebGL builds due to hard dependency on Task.

WebGL platform gating

• Explicitly excluded unsupported systems from WebGL:

  • Disk cache & file-system–based caches
  • Deep link handling
  • GLTF loading pipeline
  • Terrain rendering

• Introduced IDiskCache.Null for WebGL to preserve API shape without runtime side effects.

WebSocket / networking

• Reworked WebSocket runtime to avoid Task and thread-based synchronization.
• Replaced SemaphoreSlim and Interlocked with WebGL-safe equivalents.
• Prepared ground for JS-based WebSocket / Socket.IO bridge (follow-up work).

Authentication & bootstrap cleanup

• Simplified Web3 authentication flow:

  • Replaced IWeb3VerifiedAuthenticator with IWeb3Authenticator.
  • Removed global verification listeners; verification callbacks are now passed explicitly.

• Adjusted auto-login and bootstrap flows to the new async model.

Tests & assembly definitions

• Updated tests to conditionally include threading constructs only on non-WebGL platforms.
• Fixed asmdef references to reflect new dependencies introduced by threading abstractions.

Debugging & diagnostics

• Added explicit debug trace points to diagnose silent WebGL execution stalls.
• No behavioral changes outside WebGL; logs are temporary and scoped.

Why this change

• Prevents silent runtime hangs in WebGL caused by unsupported threading APIs.
• Enforces compile-time safety instead of relying on runtime failures.
• Establishes a clear boundary between desktop and WebGL execution models.
• Unblocks further WebGL work (JS bridges, WebWorkers, networking).

Follow-ups

• JS bridge for WebSocket / Socket.IO.
• Removal or refactor of remaining file-system–dependent systems.
• Cleanup of temporary debug logs once WebGL stability is confirmed.

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