test: add test for HLSCM::setPinnedVertices() instance API

[csparker247]

Context

HierarchicalLSCM::setPinnedVertices() is a public instance API method but has no dedicated test. The InstanceAPILevelRatio test exercises setLevelRatio/setMinCoarseVertices but not explicit pin selection.

Suggested fix

Add a test that:

1. Selects two specific boundary vertices with known positions
2. Calls hlscm.setPinnedVertices(p0, p1) and hlscm.compute(mesh)
3. Verifies those vertices land at UV (0,0) and (d,0)/(0,d) as expected by the pin placement logic

Related

Found during code review of PR #44.