feat: replace dtype with clearer precision parameter for model selection

- Changed confusing 'dtype' to 'precision' for model variant selection
- Fixed Q8 quantized model loading in transformers.js pipeline
- Added proper model file detection for q8 vs fp32 models
- Updated all references across codebase to use new parameter
- Maintains backward compatibility while providing clearer API

src/embeddings/universal-memory-manager.ts

@@ -139,7 +139,7 @@ export class UniversalMemoryManager {
       
       this.embeddingFunction = new TransformerEmbedding({
         verbose: false,
-        dtype: 'fp32',
+        precision: 'fp32',
         localFilesOnly: process.env.BRAINY_ALLOW_REMOTE_MODELS !== 'true'
       })