Add local exact-scan vector search support (runtime, API, codecs, CLI, docs, tests)

[ashione]

Motivation

• Implement a minimal local-first vector search path for fixed-dimension float vectors with exact-scan execution and common metrics (cosine/dot/l2).
• Preserve vector semantics and precision across native transport/serialization and expose vector search from C++ and Python APIs and from CLI tools.

Description

• Introduce DataType::FixedVector, extend Value to store/serialize fixed float vectors and add Value::parseFixedVector and asFixedVector accessors.
• Add a runtime vector index abstraction (vector_index.h/.cc) and an ExactScanVectorIndex implementation providing search(...) and explain(...) backed by contiguous float buffers and heap top-k selection.
• Add DataFrame APIs vectorQuery(...) and explainVectorQuery(...) and corresponding DataflowSession methods to expose vector search at the session level.
• Extend serializers and codecs: proto-like serializer and binary row-batch codec now encode FixedVector as raw float bit payloads and round-trip vector values; Arrow fast-path supports FixedSizeList<float32> columns.
• Add Python bindings for vector search (Session.vector_search, Session.explain_vector_search), Arrow fast-path parsing, and round-trip conversion of fixed vectors to Python lists.
• Provide CLIs: a Python-based single-file CLI (python_api/velaria_cli.py) and a native C++ velaria_cli example, plus scripts/build_py_cli_executable.sh to package a one-file executable.
• Add documentation (docs/local_vector_search_v01.md, README updates) describing scope, API surface, explain output, and usage examples; add example binary vector_search_benchmark and native velaria_cli example.
• Wire up BUILD files to include new sources, examples, and tests (vector_runtime_test, vector_search_benchmark, Python vector_search_test) and export the build script.

Testing

• Ran the new C++ runtime test: bazel test //:vector_runtime_test, which exercises proto-like and binary round-trip and C++ vector queries and explain, and it passed.
• Ran the new Python unit test: bazel test //python_api:vector_search_test, which validates Python API behavior, metrics, explain text and dimension mismatch handling, and it passed.

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Codex Task