Fix zero-cost edges in the TSP example

[zayenz]

The br17 example reports 87 instead of the published optimum of 39 because it treats every zero in the distance matrix as a missing edge. br17 contains valid off-diagonal zero-cost edges.

Remove that extra filtering and let circuit enforce the tour. The example then reaches 39; the two smaller instances retain costs 2276 and 22.

Checks:

• configured a Release Ninja build and built the tsp target;
• ran instances 0, 1, and 2 to optimality, obtaining 2276, 22, and 39;
• audited all bundled matrices: only br17 contains off-diagonal zero-cost edges; and
• ran git diff --check.

Closes #151.