Add build plumbing for libFuzzer+ASan fuzz build, addresses #283

- CMakeLists MCRF_FUZZER option (clang-only, -fsanitize=fuzzer-no-link)
- Makefile fuzz-build/fuzz/fuzz-long/fuzz-repro/clean-fuzz targets
- CommandLineParser -- passthrough after --exec for forwarding libFuzzer argv
- McRFPy_API: forward script_args to sys.argv in --exec mode so atheris.Setup()
  sees libFuzzer flags; set sys.argv[0] to the exec script path to match Python
  script-mode conventions
- .gitignore build-fuzz/ and corpora/crashes dirs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -30,6 +30,7 @@ option(MCRF_SANITIZE_THREAD "Build with ThreadSanitizer" OFF)
 option(MCRF_DEBUG_PYTHON "Link against debug CPython from __lib_debug/" OFF)
 option(MCRF_FREE_THREADED_PYTHON "Link against free-threaded CPython (python3.14t)" OFF)
 option(MCRF_WASM_DEBUG "Build WASM with DWARF debug info and source maps" OFF)
+option(MCRF_FUZZER "Build with libFuzzer coverage instrumentation for atheris" OFF)
 
 # Validate mutually exclusive sanitizers
 if(MCRF_SANITIZE_ADDRESS AND MCRF_SANITIZE_THREAD)
@@ -333,6 +334,15 @@ if(MCRF_SANITIZE_THREAD)
         -fsanitize=thread)
 endif()
 
+if(MCRF_FUZZER)
+    if(NOT CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+        message(FATAL_ERROR "MCRF_FUZZER=ON requires Clang. Invoke with CC=clang-18 CXX=clang++-18.")
+    endif()
+    message(STATUS "libFuzzer coverage instrumentation enabled (atheris harness)")
+    target_compile_options(mcrogueface PRIVATE -fsanitize=fuzzer-no-link)
+    target_link_options(mcrogueface PRIVATE -fsanitize=fuzzer-no-link)
+endif()
+
 # Enable Py_DEBUG when linking against debug CPython (matches pydebug ABI)
 if(MCRF_DEBUG_PYTHON OR MCRF_FREE_THREADED_PYTHON)
     target_compile_definitions(mcrogueface PRIVATE Py_DEBUG)