Adopt standalone CatHub as an optional QsoRipper dependency

[rtreit]

Parent: #540
Standalone implementation: treitforge/cathub#2

Convert QsoRipper from owning and building CatHub source to consuming CatHub as an optional standalone dependency.

Scope:

• Remove the in-tree qsoripper-cathub crate from the Rust workspace and normal build output.
• Remove CatHub-owned configuration samples, implementation documents, and duplicated runtime source.
• Preserve the unified [cat_hub] compatibility mode while supporting CatHub's standalone configuration path.
• Resolve CatHub from CATHUB_EXECUTABLE, an explicitly bundled artifact, or PATH.
• Validate the supported CatHub 0.1.x executable version before launch.
• Derive readiness from configured Hamlib NET or WinKeyer endpoints.
• Preserve normal QsoRipper logging when CatHub is absent.
• Record the external repository and supported protocol version.
• Retain and verify the temporary WinKeyer protobuf snapshot until both engines consume published CatHub protocol packages.
• Update integration scripts and documentation.

Acceptance:

• QsoRipper no longer compiles or publishes CatHub.
• Normal builds do not require a sibling CatHub checkout or network access.
• Managed unified and externally managed CatHub configurations are documented and supported.
• Launcher discovery, version validation, readiness selection, and missing-executable behavior are tested.
• Rust, .NET, Buf, dependency, coverage, and workflow-lint gates pass.

[rtreit]

The implementation also upgrades the affected GitHub Actions to official Node 24 releases. Warning annotations are treated as failed quality outcomes for this publication.