Access modifiers for grid support changed

[DGebhart777]

I would like to use a solution similar to the one in project NTv2 concerning grid support. AOT compatibility is crucial for me, which is why I can not use reflections. If we change these two access modifiers, I will fully achieve my goals.

[CLAassistant]

All committers have signed the CLA.

[fallenmi]

AI disclosure: I used OpenAI Codex to assist this review; I verified the results locally against exact head d1ab3a345c5616b0c0d348b429d84df9bf870261.

I verified the new public surface with an independent external consumer that composes a custom grid-shift MathTransform into CoordinateTransformation and ConcatenatedTransform without reflection. Exact base 94079f127 fails that consumer at compile time because the constructor and class are inaccessible; this head builds it, publishes it as macOS arm64 NativeAOT, and the native binary returns the expected transformed coordinate (11, -3) with one chain step.

The full solution test suite passes 70 tests with 4 existing skips, and the NuGet pack/package-validation path succeeds for both netstandard2.0 and netstandard2.1. The CLA check is green.

Looks good to me.