Add arxiv-complete retrieval layer (semantic + keyword)

retrieval/requirements.txt

@@ -0,0 +1,18 @@
+# arxiv-complete retrieval layer — dependencies
+#
+# Create the environment with uv (recommended) or pip:
+#
+#   uv venv && uv pip install -r requirements.txt
+#   # or
+#   python -m venv .venv && .venv/bin/pip install -r requirements.txt
+#
+# faiss-cpu is required for the ANN path.  The pure-keyword fallback (build
+# without an embedding index) only needs polars + numpy.
+
+numpy
+polars
+faiss-cpu
+
+# Only needed for the remote (production) embedding provider.
+# Comment out if you only use the local HashingEmbedder fallback.
+requests