embedding: add raw option for --embd-output-format (#16541)
* Add --embd-output-format raw for plain numeric embedding output
This new option outputs embeddings as raw space-separated floats, without JSON or 'embedding N:' prefixes. Useful for downstream vector pipelines and scripting.
* Move raw output handling into format handling section
* Move raw output handling into else-if block with other format handlers
* Use LOG instead of printf for raw embedding output
* docs: document 'raw' embedding output format in arg.cpp and README
