Add a teaching demo that separates lease expiry from fencing

[namp]

Summary

Adds a small, stdlib-only teaching demo under lease_vs_fencing/. It is isolated from the scanner driver: no scanner files are changed, and the demo does not join make check.

The demo makes two mechanisms concrete:

• A coordinator lease can be acquired, renewed, and dropped when its TTL elapses. Expiry only clears that exclusive hold. The previous holder still has the fencing token it was given.
• An effect sink accepts or rejects writes by comparing fencing tokens. FencedSink rejects a token older than one it has already observed. UnfencedSink stores every write.

The shared timeline expires Alice's lease, grants Bob a newer token, and then lets Alice write again. The fenced sink rejects that stale write. The unfenced sink stores it. A separate case writes after the TTL with no newer token presented, and the fenced sink still accepts it, so expiry is not treated as a fence.

How to run

From the repository root:

python3 -m lease_vs_fencing
python3 -m unittest lease_vs_fencing.test_lease_vs_fencing