fix: make C API compatible with macOS

Use `c_char` (signed on macOS) instead of `u8` for all FFI string
parameters in c.zig, and change the public InstrumentHooks type to an
opaque struct so the header and generated core.c agree on the type
layout. The release script now post-processes Zig's mangled struct
names into stable `struct InstrumentHooks`.

Also fixes CMakeLists using CMAKE_CXX_COMPILER_ID for a C-only project
and removes the release dependency from example build targets.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: GuillaumeLagrange

Justfile

@@ -8,13 +8,13 @@ build:
     # Use a fixed traversal seed for reproducible transpilation output
     zig build --seed 12345
 
-cmake-build-example: release
+cmake-build-example:
     cd example && mkdir -p build && cd build && cmake .. && make -j
 
 cmake-run-example: cmake-build-example
     ./example/build/example
 
-bazel-build-example: release
+bazel-build-example:
     cd example && bazelisk build //:example
 
 bazel-run-example: bazel-build-example