Expand JoinTransform with left_on/right_on/cross + add TPC-H notebook

JoinTransform now accepts either same-named (on=) or cross-named
(left_on=/right_on=) join keys, and supports how='cross'. A model
validator enforces mutual exclusion and that cross joins specify no keys.

Adds ccflow/examples/narwhals_pipelines.ipynb -- an end-to-end walkthrough
of the new abstractions on TPC-H data, refactoring Q1 into transforms,
demonstrating dependency injection, JSON serialization, multi-source
enrichment via JoinTransform, and the confluence-of-pipelines pattern
on Q3. Notebook is generated by build_notebook.py.

Signed-off-by: Pascal Tomecek <pascal.tomecek@cubistsystematic.com>

Author: ptomecek