docs: sync linear-regression notebook (seed 9 overfit + tide semantics) (#33)

Re-publish the linear regression lecture notebook after switching the
synthetic dataset to seed 9 (so the M=9 overfit is visibly dramatic) and
assigning tide semantics to x/y. Updates the registry entry.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: pantelis

notebooks/notebook-database.yml

@@ -519,6 +519,8 @@ notebooks:
   code_cells: 10
   environment: torch.dev.gpu
   description: 
+  last_executed: 2026-06-03
+  duration_seconds: 3.4
 - source: aiml-common/lectures/aleatoric-epistemic/index.ipynb
   notebook: aleatoric-epistemic/index.ipynb
   code_cells: 6