Freeze the API and prepare the 0.9 release candidate

[RejectKid]

What changed

• freezes the current native C#/.NET public surface in a compiler-enforced shipped API baseline
• enables NuGet package validation against the published OpenDisNet 0.8.0 package
• adds a project-reference-free consumer that restores the packed artifact and exercises Signal plus all 72 PDU factory/codec paths on .NET 9 and .NET 10
• runs that consumer in CI and before every release is published
• marks SemVer RC tags such as v0.9.0-rc.1 as GitHub prereleases automatically
• adds an external RC test guide, 1.0 exit criteria, and structured GitHub feedback form
• fixes the README's non-compiling named RadioId argument discovered by the package consumer

Why

The 0.9 milestone is the API freeze and external release-candidate stage. These changes turn that label into enforceable release gates: accidental public API changes fail compilation, incompatible packages fail packing, and a package that does not work for a real NuGet consumer cannot be released.

OpenDisNet remains entirely native C#/.NET; this work adds no Java code or dependency.

Consumer impact

Contributors must record intentional additive APIs in PublicAPI.Unshipped.txt. Removing or changing frozen API requires explicit breaking-change review and another RC. External testers receive a reproducible package version, validation checklist, safety guidance, and feedback form.

Validation

Clean-clone release simulation:

• locked restore passed
• generator verification passed: 233 classes and 72 PDUs
• Release build passed with 0 warnings and 0 errors
• 302 tests passed
• 0.9.0-rc.1 package and symbols package packed successfully
• package compatibility validation against 0.8.0 passed
• NuGet-only consumer passed on .NET 9 and .NET 10
• formatting and diff checks passed