feat: add explicit pin selection to AngleBasedLSCM (A5)

[csparker247]

Summary

• Extracts the core solver into private ComputeImpl(mesh, p0, p1) — no logic duplication
• Adds Compute(mesh, pin0Idx, pin1Idx) static overload for explicit pin selection
• Adds setPinnedVertices(pin0Idx, pin1Idx) + stateful pinnedVertices_ for the instance API
• Default Compute(mesh) is unchanged: auto-selects first boundary vertex + neighbor

API

// Default (unchanged behavior)
AngleBasedLSCM<float>::Compute(mesh);

// Static explicit pins
AngleBasedLSCM<float>::Compute(mesh, 0, 3);

// Instance explicit pins
AngleBasedLSCM<float> lscm;
lscm.setPinnedVertices(0, 3);
lscm.compute(mesh);

Test plan

• AngleBasedLSCM_ExplicitPins — Compute(mesh, 0, 1) produces bit-identical result to default
• AngleBasedLSCM_ExplicitPins_Reversed — Compute(mesh, 1, 0) places the correct vertex at the origin and p1 on the Y axis
• AngleBasedLSCM_SetPinnedVertices — instance API matches static explicit-pin overload
• All 72 existing tests pass

Closes #2

🤖 Generated with Claude Code