C ABI hub crate (bindings/c) foundation (#222)

## What

Introduces `wickra-c` — a `cdylib` + `staticlib` that exposes the Rust core over a **C ABI**. This is the hub every C-capable language (C, C++, Go, C#, Java, R) links against, instead of re-wiring each indicator natively. The native Python/Node/WASM bindings are untouched; this is purely additive, for ecosystems without first-class Rust tooling.

## Scope (foundation slice)

This PR deliberately validates the **whole pipeline end to end with one indicator (SMA)** before scaling to all 514, so the CI / cross-OS / header-drift mechanics are proven green first.

- Opaque `*mut T` handles; `wickra_<ind>_{new,update,batch,reset,free}`.
- NaN sentinel for warmup / NULL handles; caller-owned batch buffers; every function NULL-safe.
- cbindgen generates and commits `bindings/c/include/wickra.h` with opaque handle typedefs.
- A C smoke example (`examples/c/`) links the header + compiled library and runs (CMake + ctest).
- A `c-abi` CI job builds the library and runs the smoke test on **Linux, macOS and Windows**, plus a header drift check on Linux.

## Notes

- The per-indicator FFI blocks are plain `#[no_mangle]` functions, **not** a macro: cbindgen cannot see macro-generated functions on stable Rust (macro expansion needs nightly), so the blocks are written literally and will be generated mechanically by the ScriptHelpers `capi` wrapper in a follow-up (same model as the committed-but-generated Node `index.js`).
- `bindings/c` cannot inherit the workspace `forbid(unsafe_code)` lint (the C boundary needs raw pointers), so it mirrors every workspace lint and only relaxes `unsafe_code`. The Rust core stays `unsafe`-forbidden.

## Follow-ups (separate PRs)

- ScriptHelpers `capi` generator + wire the scalar family (~235).
- Hand-written blocks for multi-output / custom-input / bars (~279).
- Docs consistency wave (README / docs / webpage: Python·Node·WASM·Rust → +C).
- Release wiring (native-lib matrix + header/lib GH-release assets) — gated.

examples/c/README.md

@@ -0,0 +1,68 @@
+# Wickra — C / C++ examples
+
+The Wickra C ABI is a single shared/static library plus a generated header
+([`bindings/c/include/wickra.h`](../../bindings/c/include/wickra.h)). Any
+C-capable language links against the same artifact; these examples show the
+plain-C path.
+
+## Build the library
+
+From the workspace root:
+
+```sh
+cargo build -p wickra-c --release
+```
+
+This produces, in `target/release/`:
+
+| Platform | Shared library | Link target |
+|----------|----------------|-------------|
+| Linux    | `libwickra.so`     | `-lwickra` |
+| macOS    | `libwickra.dylib`  | `-lwickra` |
+| Windows (MSVC) | `wickra.dll` | `wickra.dll.lib` (import lib) |
+
+A static library (`libwickra.a` / `wickra.lib`) is emitted alongside.
+
+## Build and run the smoke example
+
+### With CMake (portable, used by CI)
+
+```sh
+cmake -S examples/c -B examples/c/build -DWICKRA_LIB_DIR="$PWD/target/release"
+cmake --build examples/c/build
+ctest --test-dir examples/c/build --output-on-failure
+```
+
+### Directly with a compiler
+
+```sh
+# Linux / macOS
+cc examples/c/smoke.c -I bindings/c/include -L target/release -lwickra -lm -o smoke
+LD_LIBRARY_PATH=target/release ./smoke        # macOS: DYLD_LIBRARY_PATH
+
+# Windows (MinGW gcc, linking the DLL directly)
+gcc examples/c/smoke.c -I bindings/c/include target/release/wickra.dll -lm -o smoke.exe
+```
+
+Expected output:
+
+```
+OK: wickra C ABI smoke passed (SMA streaming + batch + reset + NULL-safety + free)
+```
+
+## Usage shape
+
+Every indicator follows the same five-function pattern over an opaque handle:
+
+```c
+#include "wickra.h"
+
+struct Sma *sma = wickra_sma_new(14);     /* NULL on invalid params */
+double v = wickra_sma_update(sma, 42.0);  /* NaN during warmup */
+wickra_sma_reset(sma);                     /* back to fresh state */
+wickra_sma_free(sma);                      /* exactly once per _new */
+```
+
+There is no RAII across the C boundary: every `wickra_<ind>_new` must be paired
+with exactly one `wickra_<ind>_free`. All functions are NULL-safe (a NULL handle
+yields `NaN` / a no-op, never a crash).