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

.gitea/workflows/ci.yaml

@@ -42,7 +42,7 @@ jobs:
     needs: lint
     strategy:
       matrix:
-        sanitizer: [address]
+        sanitizer: [address, undefined]
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -56,6 +56,23 @@ jobs:
       - name: Unit Tests
         run: ctest --test-dir build-${{ matrix.sanitizer }} --output-on-failure
 
+  fuzz-build:
+    runs-on: ubuntu-latest
+    container: gitea.tap-tap.win/taptap/fastsync-ci:v7
+    needs: lint
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install clang
+        run: apt-get update -qq && apt-get install -y -qq clang
+
+      - name: Configure (clang + fuzz)
+        run: CC=clang CXX=clang++ cmake -B build-fuzz -S . -DENABLE_FUZZ=ON
+
+      - name: Build fuzz targets
+        run: cmake --build build-fuzz -j$(nproc)
+
   coverage:
     runs-on: ubuntu-latest
     container: gitea.tap-tap.win/taptap/fastsync-ci:v7
@@ -79,7 +96,7 @@ jobs:
       - name: Coverage Report
         run: |
           lcov --capture --directory build --output-file coverage.info --branch-coverage
-          lcov --remove coverage.info '/usr/*' '*/tests/*' --output-file coverage.info --branch-coverage --ignore-errors unused
+          lcov --remove coverage.info '/usr/*' '*/tests/*' '*/_deps/*' --output-file coverage.info --branch-coverage --ignore-errors unused
           lcov --list coverage.info
 
   valgrind: