fix: handle degenerate local G2 derivative hulls

ClumsyPilot/tests/verify_path_smoothing_local_g2_candidates.ps1

@@ -62,6 +62,10 @@ $interiorDerivative = Invoke-Scenario 'InteriorStationaryCurve'
 Assert-True $interiorDerivative.Rejected `
     'A curve with a stationary interior derivative must be rejected even when its endpoint chord is short.'
 
+$constantVelocity = Invoke-Scenario 'ConstantVelocityCurve'
+Assert-True $constantVelocity.Accepted `
+    'A nonstationary constant-velocity curve with collinear derivative controls must be accepted.'
+
 $exactSplice = Invoke-Scenario 'ExactSpliceEndpoints'
 Assert-True $exactSplice.EndpointsAreExact `
     'Splicing must write interpolated exact endpoints instead of accepting approximate candidate endpoints.'