Add GeographicLib v2.4 parity for trig accuracy, Hypot3, and Jacobi amplitude

[Copilot]

This updates the C# port to cover the user-visible GeographicLib v2.4 library delta from the upstream C++ release. The parity work focuses on the new math APIs and the exact-angle trigonometric behavior changes introduced in v2.4.

• v2.4 math parity

  • Added MathEx.Hypot3(double x, double y, double z).
  • Added EllipticFunction.Am(double x) and EllipticFunction.Am(double x, out double sn, out double cn, out double dn).

• Exact-angle trig behavior

  • Updated MathEx.Sind, MathEx.Cosd, MathEx.SinCosd, and MathEx.SinCosde to return the upstream v2.4 exact reduced-angle values for multiples of 30°, 45°, and 60°.
  • Centralized the special-case handling to keep the implementation consistent across all degree-based trig entry points.

• Regression coverage

  • Added focused tests for:
    • exact-angle Sind/Cosd/SinCosd/SinCosde results,
    • Hypot3,
    • EllipticFunction.Am special cases and consistency with Sncndn / inverse F.
  • Updated the rhumb near-pole golden value affected by the more accurate trig reduction.

• Version documentation

  • Added a 2.4.0 changelog entry in README.md summarizing the parity additions.

Example of the new v2.4 surface:

MathEx.SinCosd(45, out var s, out var c);
// s == 0.7071067811865476
// c == 0.7071067811865476

var h = MathEx.Hypot3(3, 4, 12);
// h == 13

var ell = new EllipticFunction(0.5);
var phi = ell.Am(1.25, out var sn, out var cn, out var dn);

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Pull request overview

Updates the C# GeographicLib port to align with upstream GeographicLib v2.4 math surface and exact-angle degree-based trig behavior.

Changes:

• Added MathEx.Hypot3 and EllipticFunction.Am (Jacobi amplitude) APIs.
• Updated degree-based trig entry points (Sind, Cosd, SinCosd, SinCosde) to return upstream v2.4 exact reduced-angle results for special angles.
• Added/updated regression tests and README changelog entry reflecting the v2.4 parity work.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Adds a 2.4.0 (unreleased) changelog entry describing new APIs and trig behavior fix.
GeographicLib/MathEx.cs	Introduces Hypot3 and centralized special-angle handling for degree trig functions.
GeographicLib/EllipticFunction.cs	Exposes new public Am overloads forwarding to the private implementation.
GeographicLib/EllipticFunction.Priv.cs	Implements Jacobi amplitude Am and Am(..., out sn, out cn, out dn).
GeographicLibTests/MathExTests.cs	Adds exact-angle trig assertions and Hypot3 tests.
GeographicLibTests/EllipticFunctionTests.cs	Adds tests for Am special cases and consistency with Sncndn / inverse F.
GeographicLibTests/RhumbSolveTests.cs	Updates a near-pole golden value impacted by the trig changes.

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[Copilot]

In Am, the AGM loop can exit because l reaches num_ (when GEOGRAPHICLIB_PANIC is false and the convergence test never triggers). In that case l == num_ and the subsequent a[l]/c[l] indexing will throw IndexOutOfRangeException. Please clamp l to num_ - 1 (or decrement when the loop ends via the iteration limit), or allocate a/c with length num_ + 1 to make the indexing safe on non-convergence paths.

Suggested change

	if (l >= num_)
	l = num_ - 1;