fix: harden local G2 candidate splicing

ClumsyPilot/tests/verify_path_smoothing_local_g2_candidates.ps1

@@ -58,4 +58,16 @@ Assert-True ($startBoundary.CandidateCount -gt 0) 'A start-boundary event must b
 Assert-Near 0.0 $startBoundary.StartCurvatureError 1e-8 `
     'A start-boundary candidate must use the preserved physical start vehicle curvature.'
 
+$interiorDerivative = Invoke-Scenario 'InteriorStationaryCurve'
+Assert-True $interiorDerivative.Rejected `
+    'A curve with a stationary interior derivative must be rejected even when its endpoint chord is short.'
+
+$exactSplice = Invoke-Scenario 'ExactSpliceEndpoints'
+Assert-True $exactSplice.EndpointsAreExact `
+    'Splicing must write interpolated exact endpoints instead of accepting approximate candidate endpoints.'
+
+$gearBoundary = Invoke-Scenario 'GearBoundary'
+Assert-True $gearBoundary.GearBoundaryMarkerPreserved `
+    'A window touching a gear-switch segment boundary must preserve its point-level gear marker.'
+
 Write-Output 'Path smoothing Local G2 candidate checks passed.'