Add Python bindings via PyO3/maturin

[arunmk]

Exposes Witchcraft as a Python extension module using PyO3. Each instance owns an independent reader connection (search/score) and a background indexer thread (add/remove/index/clear), mirroring the napi module's architecture.

Key details:

• src/python.rs: new PyO3 module behind the python feature flag
• build.rs: napi_build::setup() is now conditional on CARGO_FEATURE_NAPI so non-napi builds don't set conflicting linker flags
• pyo3 uses abi3-py38 stable ABI; single wheel runs on Python 3.8–3.14+
• unsafe impl Send + Sync justified by GIL: rusqlite::Connection is !Send/!Sync but only ever accessed from the Python thread
• Witchcraft::new() creates the write DB first so DB::new_reader succeeds on fresh paths; fails fast with PyRuntimeError on bad paths
• Embedder loaded once and shared via Arc between reader and indexer thread, halving peak memory (~200-500 MB saved per instance)
• Date validation: invalid ISO 8601 strings raise ValueError (consistent with UUID validation) instead of silently defaulting to now_utc()
• Indexer thread logs embed_chunks, index_chunks, add_doc, remove_doc failures via tracing::warn! instead of silently discarding them
• Drop impl joins the indexer thread so shutdown() is never required
• Makefile: python-wheel / python-dev targets with auto-detected platform features; Linux aarch64 guard added across CLI/napi/python feature sets to avoid x86-only fbgemm/hybrid-dequant on ARM
• pyproject.toml added for maturin; platform features come from Makefile
• tests/test_witchcraft.py: smoke tests with @needs_assets skip marker for tests requiring downloaded model weights
• README: Python build instructions, platform matrix, test instructions

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

[CLAassistant]

All committers have signed the CLA.