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/fuzz/fuzz_chunk_deserialize.c

@@ -2,6 +2,7 @@
 #include "data.h"
 #include <stdint.h>
 #include <stdlib.h>
+#include <string.h>
 
 int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   if (size == 0)

tests/fuzz/fuzz_compress_decompress.c

@@ -2,6 +2,7 @@
 #include "data.h"
 #include <stdint.h>
 #include <stdlib.h>
+#include <string.h>
 
 int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   if (size == 0)

tests/fuzz/fuzz_delta_deserialize.c

@@ -2,6 +2,7 @@
 #include "data.h"
 #include <stdint.h>
 #include <stdlib.h>
+#include <string.h>
 
 int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   if (size == 0)

tests/fuzz/fuzz_delta_signature_deserialize.c

@@ -2,6 +2,7 @@
 #include "data.h"
 #include <stdint.h>
 #include <stdlib.h>
+#include <string.h>
 
 int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   if (size == 0)

tests/fuzz/fuzz_metadata_from_buf.c

@@ -2,9 +2,10 @@
 #include "file.h"
 #include <stdint.h>
 #include <stdlib.h>
+#include <string.h>
 
 int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
-  if (size < sizeof(int))
+  if (size < sizeof(int) + FILE_METADATA_WIRE_SIZE)
     return 0;
 
   char* buf = malloc(size);