Fix #200: Exclude shared edges between coplanar triangles from outlines

[Copilot]

The outline detection algorithm was drawing unwanted lines between coplanar triangles that don't appear sequentially in the geometry, creating diagonal artifacts on flat surfaces.

Changes

Coplanarity Detection

• Added Triangle.AreCoplanar() to detect coplanar triangles via normal comparison
• Handles both same-direction (dot ≈ 1) and opposite-direction (dot ≈ -1) normals to account for reversed winding orders

Adjacency Calculation

• Modified Adjacency.GetAdjacencyList() to include only shared edges between coplanar triangles
• These edges are excluded from outlines (internal to flat surfaces), while non-coplanar edges remain (creases/folds)

Cross-Partition Handling

• Updated OutlineDetection.GetOutlines2() to compute global adjacency before partitioning
• Partitioning separates triangles by normal direction, so coplanar triangles with opposite normals end up in different parts
• Maps global adjacency indices to local per-part indices
• Modified Part.GetOutlines() to accept pre-calculated adjacency and handle single-triangle parts

Example

Given this geometry where triangles 1 and 3 are coplanar (z=5) but triangle 2 is angled:

// Triangle 1: horizontal at z=5
var t0 = new Triangle(
    new Point(121346, 487295, 5),
    new Point(121446, 487295, 5),
    new Point(121396, 487381.603, 5), 0);

// Triangle 2: angled (non-coplanar)
var t1 = new Triangle(
    new Point(121446, 487295, 5),
    new Point(121396, 487381.603, 5),
    new Point(121346, 487295, 100), 0);

// Triangle 3: horizontal at z=5 (coplanar with t0)
var t2 = new Triangle(
    new Point(121346, 487295, 5),
    new Point(121446, 487295, 5),
    new Point(121396, 487208.397, 5), 0);

The shared edge between t0 and t2 is now correctly excluded from the outline, while the shared edge between t0 and t1 is included.

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