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

tests/test_file.c

@@ -272,6 +272,11 @@ void test_file() {
   test_to_disk_creates_dirs();
   test_file_content_to_buffer();
   if (!getenv("FASTSYNC_UNDER_VALGRIND")) {
+    // Fork tests are skipped under valgrind because the parent process runs
+    // orders of magnitude slower than the child (parent is instrumented, child
+    // is not), which causes pipe-based protocol handshake timeouts. The parent
+    // process itself has zero valgrind errors -- the failures are all in the
+    // forked children where inherited allocations are reported as leaks.
     test_file_send_receive();
     test_file_send_no_path();
   }