[codex] Reorganize core contracts and source layers

[ashione]

What changed

This PR continues the core-convergence work by tightening the repository around five layers:

• core/logical
• core/execution
• core/contract
• interop
• experimental

It also keeps the Python ecosystem as a supported interop layer, removes the old dashboard path from the active runtime lane, and finishes the vector-search v0.1 contract tightening.

Concretely, this PR:

• adds explicit core-boundary and runtime-contract docs
• reorganizes README and Python ecosystem docs around the kernel / interop / experimental split
• adds grouped regression entrypoints for core, Python ecosystem, and experimental lanes
• moves Python demos and benchmarks into explicit examples/ and benchmarks/ areas
• aligns Python and C++ CLI vector search with the core vector contract
• hardens runtime and Python progress contract coverage in tests
• makes local CSV vector ingestion real for bracketed vectors like [1 2 3] and [1,2,3]
• adds a stable vector benchmark wrapper
• physically moves source files into src/dataflow/core, src/dataflow/interop, and src/dataflow/experimental

Why it changed

Velaria had already converged conceptually on a local runtime kernel plus supported Python ecosystem plus experimental same-host runtime lane, but the repository still mixed those concerns heavily in directory layout and target ownership.

This change makes the core boundary more explicit in both documentation and source layout so future work can expand from a clearer center instead of growing more parallel “fact cores”.

Impact

• core runtime, contract, and logical layers are easier to reason about separately
• Python remains a supported ingress and packaging surface without defining execution semantics
• actor/rpc remains available, but is visibly isolated as experimental
• vector search is now better locked to the local exact-scan contract across C++, Python, CSV, Arrow, and CLI entrypoints

Validation

• bazel test //:core_regression
• VELARIA_PYTHON_BIN=/Users/bytedance/.local/share/uv/python/cpython-3.12.10-macos-aarch64-none/bin/python3.12 ./scripts/run_python_ecosystem_regression.sh
• ./scripts/run_experimental_regression.sh
• ./scripts/run_vector_search_benchmark.sh