fix(lean,#16948): Lean-9 papermill end-to-end passage — coherent outputs AND metadata stamps

Single-tenant papermill run (kernel python3119, SemanticKernel OpenAI gpt-5.2,
USE_DEMO_MODE=false): 24/24 code cells sequential counts 1-24, 0 error, 4/4
demo banners, language_info 3.11.9, fresh metadata.papermill start/end +
per-cell metadata.execution stamps (17:50:53Z-17:57:00Z, duration 369.8s).
papermill input/output_path normalized to basename via canonical
scrub_papermill_paths.py. Replaces the jupyter_client passage whose metadata
stamps still described the 2026-09-20 OpenRouter run.

Co-Authored-By: Claude-Code <noreply@anthropic.com>

Author: jsboige