Add embedlab: interactive embedding vector playground

REPL that turns input text into L2-normalized embedding vectors with
selectable models (bge-small/base/large-zh, bge-m3), per-model
parameters, tokenizer inspection and cosine similarity.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.gitignore

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-__pycache__
+.venv/
+__pycache__/
+*.pyc