Regenerate ground-truth expected files from C# output; remove C++-mimicking implementation artifacts

[Copilot]

The test suite used expected files generated from the C++ CDT implementation. 90/990 tests failed due to tie-breaking divergence in degenerate cases — correct Delaunay output, wrong C++ parity. The fix: C# expected files generated from C# output, and removal of code that existed solely to match C++ internals.

Expected files

• All 550 .expected files regenerated from the C# triangulation
• Removed 3 orphaned files (154_*.txt) not referenced by any test assertion
• Float and double share expected files for type-independent inputs; typed inputs have separate f32_/f64_ files

Triangulation.cs — removed C++-mimicking implementations

• LLVM nth_element port (~170 lines with goto): replaced with a 20-line Lomuto quickselect
• SplitMix64 PRNG: removed entirely
  • InsertVertices_Randomized shuffle → Random.Shared Fisher-Yates
  • WalkTriangles edge offset → guard % 3
• Type-dispatch section (~120 lines, 5 wrapper methods using Unsafe.As): deleted; using static CDT.TriangleUtils added so all 17 call sites are unchanged

TriangleUtils.cs — collapsed duplicate overloads

Replaced 8 type-specific double/float overload pairs with 4 generic methods. A single private Orient2D<T> helper centralises the one unavoidable Unsafe.As dispatch (needed because Predicates has fundamentally different adaptive-precision implementations for each type). IntersectionPosition<T> now uses generic T arithmetic with no unsafe casts:

// Before: two identical bodies differing only in double vs float
public static V2d<double> IntersectionPosition(V2d<double> a, ...) { ... }
public static V2d<float>  IntersectionPosition(V2d<float>  a, ...) { ... }

// After: one generic method, Lerp expressed via T arithmetic
public static V2d<T> IntersectionPosition<T>(V2d<T> a, V2d<T> b, V2d<T> c, V2d<T> d)
    where T : unmanaged, IFloatingPoint<T>
{
    T acd = Orient2D(a, c, d);
    ...
    static T Lerp(T x, T y, T t) => (T.One - t) * x + t * y;
    return new V2d<T>(...);
}

Net: −586 / +261 lines. All 990 tests pass.

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