[v2.0] Phase 5: Testing — parity tests and benchmarks

[fsecada01]

Ensure Rust and Python backends produce identical output across all inputs. Quantify the performance gain.

Wiki: Phase 5 detail
Branch: feature/rust-backend

Tasks

• 5.1 tests/test_parity.py — happy-path parity: split_text and split_texts produce identical output from both backends
• 5.2 Edge case parity tests: empty string, no separator present, separator-only input, single chunk exceeding chunk_size
• 5.3 tests/bench_splitting.py — benchmark script: Python baseline vs Rust batch at 10,000 docs; prints speedup factor
• 5.4 All parity tests pass in CI (both use_rust=True and use_rust=False paths exercised)

Expected benchmark result

Backend	Time	Speedup
Python	~45s	1x
Rust	~1.2s	~37x