Package shared Python utilities to remove notebook path hacks

[eneskemalergin]

Summary

The notebooks currently rely on sys.path.append('../') to import shared Python utilities.

Tasks

• turn shared helpers into an importable package
• update notebooks to import the package directly
• document the supported execution location for notebooks

Acceptance criteria

• notebooks import shared code without path hacks
• imports work from the documented execution location

Why this matters

Path manipulation works locally but is fragile for outside users.