Add model metadata file

Adds metadata.yaml so the model can be listed in the AI Portal. Fields are derived from the README frontmatter and the uploaded .pte; model_size_mb is the actual artefact size. No other files touched.

metadata.yaml

@@ -0,0 +1,18 @@
+schema_version: 1.0.0
+report_type: llm-generative
+task_type: text-generation
+title: Gemma-4-E2B 8da4w — ExecuTorch
+id: Arm/gemma-4-e2b-text-executorch
+base_model_id: google/gemma-4-E2B
+profile: Arm-Optimized
+weight_dtype: int4
+model_size_mb: 2563.959
+format: executorch
+filename: gemma4_e2b_text_8da4w_emb4.pte
+quantization:
+  method: PTQ-dynamic
+  variant: 8da4w
+  weight_bits: 4
+  activation_bits: 8
+  mode: dynamic
+  weight_granularity: per-group