fix(cnc): extend first-cut edge for outside corner lead-ins

A straight lead-in at a convex outside-perimeter corner now runs along the
extension of the edge cut first, so the torch enters on that line and keeps
cutting it. The result no longer depends on which of the corner's two edges
auto-assign or the manual cursor picked, which made placement flip between
straight and 90 degrees. The approach angle is ignored at such corners.

The straight lead falls back to the first-cut edge normal when its pierce
would be closer than PierceClearance to the contour (very flat or tessellated
corners). Reflex perimeter corners bisect the notch. Line lead-outs run on
straight past a convex corner along the last-cut edge, except on tabbed
perimeters. Program generation and the Place Lead-in preview share
ResolveLeadIn/ResolveLeadOut.

docs/geometry/lead-in-placement.md

@@ -1,4 +1,4 @@
-# Lead-in placement at cutout corners
+# Lead-in placement at corners
 
 Straight (`LineLeadIn`) lead-ins at closed internal contour corners use the
 inward angle bisector instead of the normal of whichever edge was selected.
@@ -24,13 +24,39 @@ An excessively long lead-in or an approach angle rotated away from the bisector
 can still leave a small cutout. General non-circular containment/length clamping
 and sharp-corner handling for curved/composite lead-ins remain separate work.
 
+## Outside perimeter corners
+
+A straight (`LineLeadIn`) lead-in at a convex corner of the outside perimeter
+(interior angle under 180 degrees) extends the edge cut first: the pierce sits on
+that edge's line, behind the corner, and the torch travels straight into the corner
+and keeps cutting along the same line. Which of the two edges was picked (auto
+assignment or the manual cursor) does not matter; the cut direction never changes.
+The approach angle is ignored at such a corner.
+
+The straight lead is used only while its pierce stays at least
+`CuttingParameters.PierceClearance` from the contour and the lead crosses the
+contour nowhere but at the corner. Very flat corners (about 165 degrees and over for
+a 0.25 lead with 0.0625 clearance) fall back to the normal lead-in, perpendicular to
+the first-cut edge, so tessellated curves do not get straight leads. Reflex perimeter
+corners (the inside corner of an L) use the notch bisector, like cutout corners.
+
+A `LineLeadOut` mirrors this: at a convex perimeter corner it runs straight on past
+the corner along the last-cut edge, with the same clearance fallback to the last-cut
+edge's normal and a bisector at reflex corners. A tabbed perimeter keeps its old
+lead-out. `ContourCuttingStrategy.ResolveLeadIn`/`ResolveLeadOut` own these rules;
+program generation and the manual preview share them. Other lead-in styles are
+unchanged.
+
 ## Regression checks
 
-Run `dotnet test OpenNest.Tests/OpenNest.Tests.csproj --filter FullyQualifiedName~CutoutCornerLeadInTests`.
-The tests exercise generated part programs, default automatic placement, every
+Run `dotnet test OpenNest.Tests/OpenNest.Tests.csproj --filter "FullyQualifiedName~CutoutCornerLeadInTests|FullyQualifiedName~PerimeterCornerLeadInTests"`.
+The cutout tests exercise generated part programs, default automatic placement, every
 rectangular corner with both adjoining edges and windings, part rotation, acute
 and obtuse angles, reflex corners, line/arc corners, preview agreement, and
-unchanged/fallback behavior.
+unchanged/fallback behavior. The perimeter tests cover every square corner under both
+windings and a rotation, auto and manual placement from either edge, preview
+agreement, approach-angle handling, the flat-corner clearance fallback, reflex
+notches, and straight lead-outs.
 
 A headless before/after import of `4980 A01 PT07.dxf` (SHA-256
 `1535D77BC1EEEDD21A27E7CE91EA4C51055118D019C5A09C144F1F41740895B6`)