feat(core): add MLflow tracking helper with disable env-flag

feat(core): extract write_parquet() helper for §6 storage contract

feat(core): extract pin_threads() helper for determinism

fix(core): apply level + propagate every call; tolerate pre-attached handlers

test(core): capture handler stream directly instead of monkey-patching caplog

feat(core): add shared structured logger with idempotent handler attach

chore: bootstrap NeuroBridge project skeleton