v0.3.4 — schema-enforced results

Executor upgrade (#91), born from a live incident where a correct decomposition arrived wrapped in prose + markdown fences and mis-routed as a raw-text candidate:

• Native --json-schema enforcement: on Claude Code ≥ 2.1.205 the executor constrains the model's output to the result contract — the answer arrives schema-valid in structured_output. Version support probed via a free claude --version call; the retry path provably cannot double-spend.
• Extraction ladder fallback for older CLIs: strict parse → last balanced top-level JSON object (string-aware scanner, envelope-shaped preferred) → fenced blocks → keep-and-charge raw text. Never throws.
• parse_mode recorded on every result for diagnosability.

423 tests.