fix: address PR review — fuzz target fixes, UBSan, CI improvements

Critical fixes:
- Add missing #include <string.h> to 5 fuzz targets (wouldn't compile with GCC14+/Clang16+)
- Fix fuzz_metadata_from_buf.c OOB read: guard size >= sizeof(int) + FILE_METADATA_WIRE_SIZE
- Add Clang compiler check for ENABLE_FUZZ in CMakeLists.txt (fail fast at configure time)
- Add fuzz-build CI job (install clang, build all 6 fuzz targets with ENABLE_FUZZ=ON)

Warning fixes:
- Add UBSan to sanitizer CI matrix (address + undefined)
- Remove tautological test_property_glob_consistency (pure/deterministic function)
- Use fixed seed srand(42) instead of srand(time(NULL)) for reproducible property tests
- Add valid-header + truncated-instructions delta robustness test (exercises instruction-loop error paths)
- Fix lcov --remove to exclude '*/_deps/*' (xxhash coverage pollution)
- Add comment explaining FASTSYNC_UNDER_VALGRIND skip in test_file.c
- Update .gitignore for build-*/ directories

CMakeLists.txt

@@ -9,8 +9,8 @@ set(CMAKE_C_STANDARD_REQUIRED ON)
 add_compile_options(-Wall -g -O3)
 
 # --- Sanitizer option ---
-set(SANITIZER "none" CACHE STRING "Sanitizer to enable (address, thread, none)")
-set_property(CACHE SANITIZER PROPERTY STRINGS address thread none)
+set(SANITIZER "none" CACHE STRING "Sanitizer to enable (address, thread, undefined, none)")
+set_property(CACHE SANITIZER PROPERTY STRINGS address thread undefined none)
 
 if(SANITIZER STREQUAL "address")
 	add_compile_options(-fsanitize=address -fno-omit-frame-pointer -g)
@@ -18,8 +18,11 @@ if(SANITIZER STREQUAL "address")
 elseif(SANITIZER STREQUAL "thread")
 	add_compile_options(-fsanitize=thread -fno-omit-frame-pointer -g)
 	add_link_options(-fsanitize=thread)
+elseif(SANITIZER STREQUAL "undefined")
+	add_compile_options(-fsanitize=undefined -fno-omit-frame-pointer -g)
+	add_link_options(-fsanitize=undefined)
 elseif(NOT SANITIZER STREQUAL "none")
-	message(FATAL_ERROR "Unknown sanitizer: ${SANITIZER}. Supported values: address, thread, none")
+	message(FATAL_ERROR "Unknown sanitizer: ${SANITIZER}. Supported values: address, thread, undefined, none")
 endif()
 
 # --- Strict warnings option ---
@@ -84,7 +87,10 @@ add_test(NAME unit_all COMMAND tests)
 # --- Fuzz targets (requires clang) ---
 option(ENABLE_FUZZ "Build fuzz targets (requires clang)" OFF)
 if(ENABLE_FUZZ)
-	file(GLOB FUZZ_SRCS "tests/fuzz/*.c")
+  if(NOT CMAKE_C_COMPILER_ID MATCHES "Clang")
+    message(FATAL_ERROR "ENABLE_FUZZ requires Clang (compiler is ${CMAKE_C_COMPILER_ID})")
+  endif()
+  file(GLOB FUZZ_SRCS "tests/fuzz/*.c")
 	foreach(FUZZ_SRC ${FUZZ_SRCS})
 		get_filename_component(FUZZ_NAME ${FUZZ_SRC} NAME_WE)
 		add_executable(${FUZZ_NAME} ${FUZZ_SRC} ${SHARED_SRCS})