supportedExtensions() returned its static set by value, so capabilities()
constructed the extension vector from begin() of one temporary and end()
of another. That range is undefined behavior. libstdc++ and libc++ release
iterators happened to walk the first copy to completion, but the MSVC debug
STL rejects it in _Verify_range with a modal "Microsoft Visual C++ Runtime
Library" assertion dialog. HandlerRegistry::selectBest calls capabilities()
for every candidate, so every lines: search and every applyExtractScope call
hung forever on Windows Debug builds.
Return the shared set by const reference and take begin()/end() from that
one instance. New text_basic_handler unit tests pin the shared instance,
the capability list, registry selection and line-range extraction; the old
code also fails them under -D_GLIBCXX_DEBUG with "function requires a valid
iterator range".
Signed-off-by: trvon <me@trevon.dev>
