Add TextSpitter 2.0 Rust/PyO3 core with full test suite

Introduces a Maturin-based Rust extension (_core.pyd) that accelerates
encoding detection, text normalization, token counting, and token-aware
chunking, while keeping a pure-Python fallback path (_fallback.py) for
environments where the compiled extension is unavailable.

Rust crates: pyo3 0.21 (Bound API), chardetng, tiktoken-rs 0.5, rayon,
unicode-normalization, regex. All batch methods release the GIL via
py.allow_threads so CPython threads stay unblocked during heavy work.

Python layer changes:
- __init__.py: try-import _core, fall back to _fallback, export
  _RUST_AVAILABLE flag alongside Chunk, TextChunker, TextNormalizer,
  TokenCounter, detect_encoding
- core.py: uses detect_encoding() instead of the old four-attempt loop
- _fallback.py: tiktoken-backed fallback with model validation at
  construction time; word-based middle-truncation when tiktoken absent
- pyproject.toml: switched build-backend to maturin; added tiktoken to
  dev deps; added [tool.maturin] config; expanded .gitignore for Rust

Tests added (239 total, all green):
- test_detect_encoding.py   -- both Rust and fallback paths
- test_normalizer.py        -- Unicode forms, whitespace, OCR, headers
- test_token_counter.py     -- count, batch, truncate, parallel stress
- test_chunker.py           -- field correctness, max_tokens, tables,
                               section titles, batch, parallel stress
- test_rust_integration.py  -- end-to-end pipeline and interface parity

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

Author: fsecada01

.gitignore

@@ -7,4 +7,15 @@ labs
 .venv
 *.7z
 *.env
-*.egg*/
+*.egg*/
+
+# Rust / Maturin build artifacts
+target/
+*.pyd
+*.pdb
+*.so
+
+# IDE / tool state
+.idea/
+.serena/
+claudedocs/