feat: Phase 32 — Rust search engine core (codexa-core) with full integration

- Add codexa-core Rust crate: native VectorStore (rayon-parallel inner-product),
  BM25 index (identical tokenisation), code chunker, file scanner (blake3),
  and RRF hybrid fusion — all exposed via PyO3 bindings
- Add rust_backend.py: unified Python bridge with automatic fallback detection
- Integrate Rust backend into VectorStore: cached RustVectorStore mirror
  for accelerated search, dual-format save (vectors.bin + vectors.faiss)
- Integrate Rust BM25 into keyword_search: transparent RustBM25Index
  dispatch with in-memory and disk caching
- Integrate Rust RRF into hybrid_search: native reciprocal_rank_fusion_rs
  with Python fallback
- Update search_service: detect both vectors.faiss and vectors.bin for
  auto-indexing
- Update .gitignore: exclude codexa-core/target/ build artifacts
- All 2596 existing tests pass — zero regressions

Author: M9nx

codexa-core/Cargo.toml

@@ -0,0 +1,28 @@
+[package]
+name = "codexa-core"
+version = "0.1.0"
+edition = "2021"
+description = "Rust-native search and indexing engine for CodexA"
+license = "MIT"
+
+[lib]
+name = "codexa_core"
+crate-type = ["cdylib"]
+
+[dependencies]
+pyo3 = { version = "0.22", features = ["extension-module"] }
+numpy = "0.22"
+rayon = "1"
+blake3 = "1"
+walkdir = "2"
+globset = "0.4"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+regex = "1"
+ignore = "0.4"
+memmap2 = "0.9"
+
+[profile.release]
+opt-level = 3
+lto = "thin"
+codegen-units = 1

codexa-core/pyproject.toml

@@ -0,0 +1,17 @@
+[build-system]
+requires = ["maturin>=1.0,<2.0"]
+build-backend = "maturin"
+
+[project]
+name = "codexa-core"
+version = "0.1.0"
+description = "Rust-native search and indexing engine for CodexA"
+requires-python = ">=3.9"
+license = { text = "MIT" }
+classifiers = [
+    "Programming Language :: Rust",
+    "Programming Language :: Python :: Implementation :: CPython",
+]
+
+[tool.maturin]
+features = ["pyo3/extension-module"]