New context_new.rs with the AST-based context window design:
- AstNode: role + NodeBody (Leaf with text+token_ids, or Branch with children)
- Tokens only on leaves, branches walk children
- render() produces UTF-8, tokenize produces token IDs, same path
- ResponseParser state machine for streaming assistant responses
- Role enum covers all node types including sections
Still needs: fix remaining pattern match issues, add ContextState wrapper,
wire into mod.rs, replace old context.rs.
Does not compile yet — this is a design checkpoint.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
