fix: add immutable libmysqlclient revision 8.4.6.1

[wellwei]

Summary

• Append compat.libmysqlclient@8.4.6.1 as an immutable package revision.
• Keep the existing 8.4.6 index entry unchanged; update Connector/C++ 26.7.0's client dependency and consumer smoke test to use the repaired revision.
• Preserve Connector/C++ as one Form B package that builds both X DevAPI and JDBC.

Boundary Constraints

• libmysqlclient is a client-only extraction from the MySQL Server 8.4.6 source. It is Form A and builds libmysqlclient.a from checked-in sources with mcpp. It does not ship MySQL Server, CMake, the command-line client, or prebuilt libraries.
• 8.4.6.1 is a package revision of the same MySQL 8.4.6 client ABI. The old 8.4.6 URL/SHA entry is retained and is not rewritten.
• Connector/C++ remains a single Form B package. Its install hook builds and installs both X DevAPI and JDBC, and it requires the source-built libmysqlclient, OpenSSL, zlib, zstd, CMake, and platform system libraries.
• The temporary MySQL client probe archive is created only for Connector/C++ CMake detection; it is not installed or used as the final MySQL client library.
• Package entries and consumer tests are intentionally limited to Linux and macOS. Windows is not published by this change; a Windows workspace pass does not imply Windows package support.
• This PR does not modify compat.openssl. Static OpenSSL objects on macOS may retain the existing host/deployment-target constraint; that risk remains an integration boundary for consumers.
• The extracted sources and Connector/C++ remain subject to their upstream GPL-2.0-only and Universal FOSS Exception licensing terms.

Validation

• mcpp 2026.8.8.2 xpkg parse passed for both changed descriptors.
• Lua syntax, mirror URL, package-name, cross-package-reference, and platform-version checks passed.
• macOS consumer tests passed with the pinned mcpp: libmysqlclient 1/1 and Connector/C++ 1/1, including X DevAPI and JDBC headers/linkage.
• Downloaded 8.4.6.1.tar.gz SHA-256: e50e5da37baa26fff56952d85bd8e33b577be7a534e67df0b8e10a5cc8507af8.
• Linux cold-install verification is intentionally left to the PR matrix; cache hits are not treated as that proof.