P3-A: WebGL Embedding Provider

[github-actions]

Why: Explicit webgl fallback for systems without WebGPU/WebNN.

• P3-A1: Implement embeddings/OrtWebglEmbeddingBackend.ts

  • Use ONNX Runtime Web with explicit webgl execution provider
  • Implement EmbeddingBackend interface
  • Add to ProviderResolver candidate sets

• P3-A2: Add WebGL provider test coverage

  • tests/embeddings/OrtWebglEmbeddingBackend.test.ts

Exit Criteria: webgl backend available for systems without WebGPU.