Pages: add per-cell progress log to diagnose SIGSEGV in demo.py subprocess

Each cell now prints its number and first-line preview to the real stdout
(bypassing the capture buffer) before exec-ing.  This survives hard crashes
and will show us exactly which cell triggers the SIGSEGV in demo.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: larsgeb

scripts/render_notebooks.py

@@ -82,6 +82,10 @@ def _patched_show(*_args, **_kwargs) -> None:
         if not src:
             continue
 
+        # Progress line bypasses stdout_buf so it survives a crash mid-cell.
+        preview = src.splitlines()[0][:60]
+        print(f"  [cell {exec_count}] {preview}", file=sys.__stdout__, flush=True)
+
         cell_images.clear()
         stdout_buf = io.StringIO()