# 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.
With the default 90-degree approach angle, a rectangular cutout therefore gets
a diagonal lead-in into the scrap, rather than one lying along the other edge.
This applies to automatic assignment and manual placement. The manual preview
uses the same Core calculation, including when the lead-in style changes while
the cursor is stationary.

`ContourCuttingStrategy.ComputeLeadInNormal` combines the two adjacent inward
unit normals. It handles either winding, either selected edge, rotated parts,
and line/arc junctions using their local normals. Summing vectors avoids angle
wraparound and edge-length weighting. Open contours, disconnected endpoints,
zero-length edges, and cusps without a unique bisector retain the entity normal.

The configured approach-angle offset is still applied relative to the computed
normal; 90 degrees follows the bisector. Lead-in length is unchanged. Mid-edge
points, external contours, circles, curved/composite lead-in styles, and
lead-outs keep their existing placement rules.

This is a local direction correction, not a whole-path clearance guarantee.
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|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. 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`)
reproduced the issue on all five 0.282 × 0.532 rectangular cutouts. At the default
0.125 lead-in length, all five corrected pierce points are approximately 0.08838835
inside both adjacent edges. The straight segments stay inside the rectangles
except for their contour endpoints; the other two generated lead-ins remain
unchanged. Coordinates were compared with a 1e-8 tolerance to allow incremental
program round-trip floating-point noise. The source drawing is not bundled.

Windows manual acceptance: import the part and use Plate > Assign Lead-ins.
Confirm diagonal lead-ins at all five small rectangular cutouts. Under Plate >
Place Lead-in, select the part, lock a cutout, and hover a corner: the preview
should point into the cutout and the committed lead-in should match. Hover a
mid-edge point and confirm perpendicular placement is unchanged. Windows visual
interaction is not verified by the Linux cross-build.