Run v3 C tests in GitHub CI

ci.yml: two new jobs.
  - v3-c-tests builds tester_v3_c (security regressions) and
    tester_v3_c_v1port (18 ported v1 unit tests) with the default
    system cc (gcc on ubuntu-latest) and runs both.
  - v3-c-tests-sanitizers rebuilds the same suites under clang with
    -fsanitize=address,undefined and ASAN_OPTIONS=halt_on_error=1 plus
    leak detection so memory-safety regressions break CI.

c-cpp.yml: add a v3_c_tests step to build-linux so the legacy gcc
workflow also exercises the v3 C parser end-to-end.

Both invoke the parser via parse_auto with TINYGLTF3_ENABLE_FS so the
external-file paths (and the new path-traversal/file-size guards)
are exercised end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/c-cpp.yml

@@ -100,6 +100,17 @@ jobs:
         ./tester_noexcept
         cd ..
 
+    - name: v3_c_tests
+      run: |
+        cd tests
+        cc -I../ -std=c11 -g -O0 -DTINYGLTF3_ENABLE_FS \
+           -o tester_v3_c tester_v3_c.c ../tiny_gltf_v3.c
+        ./tester_v3_c
+        cc -I../ -std=c11 -g -O0 -DTINYGLTF3_ENABLE_FS \
+           -o tester_v3_c_v1port tester_v3_c_v1port.c ../tiny_gltf_v3.c
+        ./tester_v3_c_v1port
+        cd ..
+
 
   build-rapidjson-linux: