Clean up source builds and native registration

[xjsun1221]

Problem

This PR addresses three source-build and native-interface issues:

1. The repository contains compiled objects, Rfast.dll, and symbols.rds
   under src/. A direct installation from the GitHub archive can therefore
   skip compilation and reuse a DLL built against a different TBB ABI.
2. cts_rf.cpp and g2t.h define different global classes with the same
   TestResult name, violating the C++ one-definition rule.
3. The package uses hand-written wrappers and init.c registration, but also
   retains stale Rcpp::export attributes. Running compileAttributes() can
   generate duplicate bhattacharyya wrappers and unregistered _Rfast_*
   entry points.

Change

• Remove and ignore platform-specific build artifacts.
• Rename the cts-local result type to CtsTestResult.
• Remove stale automatic-export attributes while preserving the existing
  hand-written registration and public R API.

Validation

• Windows 11 x64
• R 4.5.0
• Rtools45 / GCC 14.2.0
• RcppParallel 6.2.0
• Reproduced the original Windows LoadLibrary failure after make reported
  Nothing to be done for 'all' for the unmodified GitHub archive.
• Rcpp::compileAttributes() completes without warnings and does not create
  conflicting wrapper files.
• Clean source compilation, installation, and loading succeeded.
• Smoke tests passed for categorical pc.skel(), g2Test(), coeff(),
  eachcol.apply(), and vecdist().
• R CMD check --no-manual completed with 0 errors. Its only warning was that
  locally installed dependencies were built under newer R 4.5 patch releases;
  the unavailable suggested package philentropy was not forced.