Add C API

C-API.md

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+# libsoliton_capi
+
+C ABI FFI layer for [libsoliton](https://git.lo.sh/lo/libsoliton) — a pure-Rust post-quantum cryptographic library.
+
+## API
+
+The complete API is defined in [`soliton.h`](soliton.h) (48 exported functions, cbindgen-generated). All functions return `int32_t` error codes; opaque handles are created/freed via matched `_new`/`_free` pairs.
+
+## Build
+
+```bash
+cargo build --release -p libsoliton_capi
+```
+
+Produces:
+- `target/release/libsoliton_capi.so` (Linux) / `.dylib` (macOS) / `.dll` (Windows)
+- `target/release/libsoliton_capi.a` (static)
+
+## Link
+
+**pkg-config:**
+```bash
+gcc main.c $(pkg-config --cflags --libs libsoliton) -o main
+```
+
+**CMake:**
+```cmake
+find_package(Soliton REQUIRED)
+target_link_libraries(myapp Soliton::Soliton)
+```
+
+**Direct:**
+```bash
+gcc main.c -I path/to/soliton_capi -L path/to/target/release -lsoliton_capi -o main
+```