[v2.0] Phase 2: Core Rust implementation

[fsecada01]

Implement the Rust splitting engine exposed to Python via PyO3.

Wiki: Phase 2 detail
Branch: feature/rust-backend

Tasks

• 2.1 src/lib.rs — PyO3 module entry point; register CharacterTextSplitter and TokenTextSplitter
• 2.2 src/splitters/character.rs — CharacterTextSplitter with split_text + split_texts (Rayon parallel); split_internal in separate impl block (not exposed to Python)
• 2.3 src/splitters/token.rs — TokenTextSplitter with token counting and chunking
• 2.4 src/splitters/recursive.rs — RecursiveSplitter with configurable separator hierarchy
• 2.5 src/utils.rs — shared Rust utilities (encoding helpers, etc.)
• 2.6 cargo clippy --all-targets -- -D warnings passes; cargo fmt --check passes

Key constraint

split_internal must live in a separate impl block (no #[pymethods]) so PyO3 does not expose it to Python.