Rename ring_buffer target to ring_buffer-lib and gate demo executable
- Rename static library target from "ring_buffer" to "ring_buffer-lib"
- Add LIB_DEMO_ENABLE option (default OFF) to conditionally build the demo executable
- Remove redundant enable_language(C CXX) call (already covered by project() LANGUAGES)
- Simplify include directory variables and drop unused SOURCES/INCLUDE_DIRS intermediate variables