fix: stabilize quintic decimal knot spacing

ClumsyPilot/tests/verify_path_smoothing_quintic.ps1

@@ -297,6 +297,31 @@ Assert-Equal 'Failed' (Get-PropertyValue $shortCandidate 'Status').ToString() 'A
 Assert-True (-not (Get-PropertyValue $shortCandidate 'Succeeded')) 'A degenerate short quintic segment must not be executable.'
 Assert-Equal 0 (Get-PropertyValue $shortCandidate 'Segments').Count 'A terminal quintic degeneracy must publish no geometry.'
 
+# Repeated decimal addition must not create a spurious 1e-16 m terminal residual when the
+# requested spacing divides the local length exactly. A real 0.005 m remainder remains below
+# the 0.01 m minimum and must still fail terminally.
+$decimalMultipleSource = @(
+    (New-Point 0.0 0.0 0.0 0.0),
+    (New-Point 1.0 0.0 1.0 0.0))
+$decimalMultipleCandidate = Invoke-Candidate @((New-DirectionSegment 0 $forward $decimalMultipleSource)) 0.0 0.10 0.01
+Assert-Equal 'Success' (Get-PropertyValue $decimalMultipleCandidate 'Status').ToString() 'Decimal-exact knot multiples must not become a terminal under-minimum residual failure.'
+$decimalMultipleOutput = @(Get-PropertyValue $decimalMultipleCandidate 'Segments')[0].Points
+Assert-Equal 81 $decimalMultipleOutput.Count 'A 1.0 m segment with 0.1 m spacing must create exactly ten valid quintic intervals.'
+Assert-Near 1.0 $decimalMultipleOutput[$decimalMultipleOutput.Count - 1].ArcLength 0.0 'Decimal-multiple knot normalization must retain the exact terminal arc length.'
+
+$meaningfulShortResidualSource = @(
+    (New-Point 0.0 0.0 0.0 0.0),
+    (New-Point 1.005 0.0 1.005 0.0))
+$meaningfulShortResidualCandidate = Invoke-Candidate @((New-DirectionSegment 0 $forward $meaningfulShortResidualSource)) 0.0 0.10 0.01
+Assert-Equal 'Failed' (Get-PropertyValue $meaningfulShortResidualCandidate 'Status').ToString() 'A genuine 0.005 m terminal residual must remain a terminal spacing failure.'
+Assert-Equal 0 (Get-PropertyValue $meaningfulShortResidualCandidate 'Segments').Count 'A genuine under-minimum residual must publish no geometry.'
+
+$smallScaleResidualSource = @(
+    (New-Point 0.0 0.0 0.0 0.0),
+    (New-Point 0.000000000000105 0.0 0.000000000000105 0.0))
+$smallScaleResidualCandidate = Invoke-Candidate @((New-DirectionSegment 0 $forward $smallScaleResidualSource)) 0.0 0.00000000000001 0.000000000000006
+Assert-Equal 'Failed' (Get-PropertyValue $smallScaleResidualCandidate 'Status').ToString() 'Endpoint normalization tolerance must scale with local arc magnitude and preserve a genuine small residual failure.'
+
 # Local-arc reference mapping, rather than sample index/global distance, must reject this unsafe nonuniform path.
 $nonuniformUnsafeSource = @(
     (New-Point 0.0 0.0 0.0 0.0 0.50),